He signed an open letter as one of 250 celebrities saying Britain would be culturally better off in the EU.
On 23 June, the UK will settle a question that's been rumbling close to the surface of British politics for a generation: should the country remain within the European Union, or leave the organisation and go it alone.
Both sides insist that the outcome of the vote will settle the matter of Britain's EU membership for the foreseeable future.
The Conservative election victory last year activated a manifesto pledge to hold an in/out referendum on Britain's membership of the European Union by the end of 2017.
David Cameron made the promise of an EU referendum at a time when he was under pressure from Eurosceptic backbenchers within his own party – and when the Tories appeared to be losing votes to Ukip. Most political commentators agree that, given a free hand, he would not have wanted a referendum, and that he is now desperate to secure Britain's place in the EU.
This winter, he embarked on a tour of EU capitals as he sought to renegotiate Britain's terms of membership, which concluded at a summit in February. Presenting the result as a victory, he vowed to campaign with his "heart and soul" to keep Britain inside a "reformed" EU, but several members of his own Cabinet are campaigning for a British exit – or Brexit.
So what are the advantages and disadvantages of being a part of Europe? Would Britain be better off staying inside the club or going it alone?
The pros and cons of leaving the EU
The greatest uncertainty associated with leaving the EU is that no country has ever done it before, so no one can predict the exact result. Nevertheless, many have tried.
Leaving the EU would result in an immediate cost saving, as the country would no longer contribute to the EU budget. Last year, Britain paid in £13bn, but it also received £4.5bn worth of spending, says Full Fact "so the UK's net contribution was £8.5bn". That's about 7 per cent of what the Government spends on the NHS each year.
What's harder to determine is whether the financial advantages of EU membership, such as free trade and inward investment (see below) outweigh the upfront costs.
The EU is a single market in which no tariffs are imposed on imports and exports between member states. "More than 50 per cent of our exports go to EU countries," says Sky News, "and our membership allows us to have a say over how trading rules are drawn up."
Britain also benefits from trade deals between the EU and other world powers. "The EU is currently negotiating with the US to create the world's biggest free trade area," says the BBC, "something that will be highly beneficial to British business."
Britain risks losing some of that negotiating power by leaving the EU, but it would be free to establish its own trade agreements.
Ukip leader Nigel Farage believes Britain could follow the lead of Norway, which has access to the single market but is not bound by EU laws on areas such as agriculture, justice and home affairs. But others argue that an "amicable divorce" would not be possible.
"If Britain were to join the Norwegian club," says The Economist, "it would remain bound by virtually all EU regulations, including the working-time directive and almost everything dreamed up in Brussels in future." And it would no longer have any influence on what those regulations said.
Leading Brexit campaigner Boris Johnson, meanwhile, has proposed adopting a Canada-style trade arrangement. "I think we can strike a deal as the Canadians have done based on trade and getting rid of tariffs" and have a "very, very bright future", he said. The idea was quickly dismissed by the PM, who said it would mean "years of painful negotiations and a poorer deal than we have today".
Eurosceptics argue that the vast majority of small and medium sized firms do not trade with the EU but are restricted by a huge regulatory burden imposed from abroad.
A study by the think-tank Open Europe, which wants to see the EU radically reformed, found that the worst-case "Brexit" scenario is that the UK economy loses 2.2 per cent of its total GDP by 2030 (by comparison, the recession of 2008-09 knocked about 6 per cent off UK GDP). However, it says that GDP could rise by 1.6 per cent if the UK was able to negotiate a free trade deal with Europe – ie to maintain the current trade set-up – and pursued "very ambitious deregulation".
Whether other EU countries would offer such generous terms is one of the big unknowns of the debate. Pro-exit campaigners argue that it would be in the interests of other European countries to re-establish free trade, but their opponents suggest that the EU will want to make life hard for Britain in order to discourage further breakaways.
France has said recently that there would be "consequences" for Britain if it left the EU.
Inward investment is likely to slow in the run-up to the vote, due to the uncertainty of the outcome and its consequences: that's what happened in before the Scottish independence referendum in 2014.
In the longer term, there are diverging views: pro-Europeans think the UK's status as one of the world's biggest financial centres will be diminished if it is no longer seen as a gateway to the EU for the likes of US banks, while Brexit campaigners suggest that, free from EU rules a regulations, Britain could reinvent itself as a Singapore-style supercharged economy.
Fears that car-makers could scale back or even end production in the UK vehicles could no longer be exported tax-free to Europe were underlined by BMW's decision to remind its UK employees at Rolls-Royce and Mini of the "significant benefit" EU membership confers. Likewise, Business for New Europe says tax revenues would drop if companies that do large amounts of business with Europe – particularly banks – moved their headquarters back into the EU.
Barclays, however, has put forward a worst-case scenario that might benefit the Outers. It says the departure of one of the EU's most powerful economies would hit its finances and boost populist anti-EU movements in other countries. This would open a "Pandora's box", says the Daily Telegraph, which could lead to the "collapse of the European project".
The UK would then be seen as a safe haven from those risks, attracting investors, boosting the pound and reducing the risk that Scotland would "leave the relative safety of the UK for an increasingly uncertain EU".
Under EU law, Britain cannot prevent anyone from another member state coming to live in the country – while Britons benefit from an equivalent right to live and work anywhere else in the UK. The result has been a huge increase in immigration into Britain, particularly from eastern and southern Europe.
According to the Office for National Statistics, there are 942,000 eastern Europeans, Romanians and Bulgarians working in the UK, along with 791,000 western Europeans – and 2.93m workers from outside the EU. China and India are the biggest source of foreign workers in the UK.
Inners say that, while the recent pace of immigration has led to some difficulties with housing and service provision, the net effect has been overwhelmingly positive. By contrast, Farage says immigration should be cut dramatically, and the leaving the EU is the only way to "regain control of our borders". Other pro-Brexit campaigners would not necessarily reduce immigration, but say that it should be up to the British Government to set the rules.
David Cameron says that concessions he won during the renegotiation of Britain's EU membership will reduce immigration as new arrivals will receive a lower rate of child benefit.
The effect of leaving the EU on British jobs depends on a complex interplay of the factors above: trade, investment and immigration.
Pro-EU campaigners have suggested that three million jobs could be lost if Britain goes it alone. However, while "figures from the early 2000s suggest around three million jobs are linked to trade with the European Union," says Full Fact, "they don't say they are dependent on the UK being an EU member."
If trade and investment fell post-Brexit, then some of these jobs would be lost – but if they rose, then new jobs would be created.
A drop in immigration would, all else being equal, mean more jobs for the people who remained, but labour shortages could also hold back the economy, reducing its potential for growth.
Stuart Rose, former Marks & Spencer chief executive and a prominent pro-EU campaigner, conceded recently that wages may rise if Britain leaves – which would be good for workers, but less so for their employers.
Writing for the London School of Economics, Professor Adrian Favell says limiting freedom of movement would deter the "brightest and the best" of the continent from coming to Britain and reduce the pool of candidates employers can choose from.
Free movement of people across the EU also opens up job opportunities for British workers seeking to work elsewhere in Europe.
Britain's place in the world
For Outers, leaving the EU will allow Britain to re-establish itself as a truly independent nation with connections to the rest of the world. To Inners, Brexit would result in the country giving up its influence in Europe, turning back the clock and retreating from the global power networks of the 21st century.
Brexit would bring some clear-cut advantages, says The Economist. The UK "would regain control over fishing rights around its coast", for example. But it concludes that the most likely outcome is that Britain would find itself "a scratchy outsider with somewhat limited access to the single market, almost no influence and few friends".
Britain would remain a member of Nato and the UN, but it may be regarded as a less useful partner by its key ally, the US. The American government fears that the "EU referendum is a dangerous gamble that could unravel with disastrous consequences for the entire continent", says The Guardian.
Work and Pensions Secretary Iain Duncan Smith, who has come out in favour of Brexit, says we are leaving the "door open" to terrorist attacks by remaining in the EU. "This open border does not allow us to check and control people," he says.
However, a dozen senior military figures, including former chiefs of defence staff Lord Bramall and Jock Stirrup, say the opposite. In a letter released by No 10, they argue that the EU is an "increasingly important pillar of our security", especially at a time of instability in the Middle East and in the face of "resurgent Russian nationalism and aggression".
Defence Secretary Michael Fallon has also said the UK benefits from being part Europe, as well as Nato and the United Nations. "It is through the EU that you exchange criminal records and passenger records and work together on counter-terrorism," he said. "We need the collective weight of the EU when you are dealing with Russian aggression or terrorism."
In contrast, Colonel Richard Kemp, writing in The Times, says these "critical bilateral relationships" would persist regardless of membership, and that it is "absurd" to suggest that the EU would put its own citizens, or the UK's, at greater risk by reducing cooperation in the event of Brexit.
"By leaving, we will again be able to determine who does and does not enter the UK," says Kemp, a former head of the international terrorism team at the Cabinet Office. "Failure to do so significantly increases the terrorist threat here, endangers our people and is a betrayal of this country."
"The prospect of linking arms with Nigel Farage and George Galloway and taking a leap into the dark is the wrong step for our country."
EU referendum: the big questions for Britain
When will the EU referendum be held?
The date has been set for 23 June 2016.
What will the referendum ask?
The Conservatives recommended that the question should be: "Do you think that the United Kingdom should remain a member of the European Union?" However, the government bowed to pressure from the Electoral Commission after concerns that the phrasing of the question might be seen as biased towards those campaigning to remain a part of the union. The wording has since been changed to:
Should the UK remain a member of the EU or leave the EU?
Who can vote in the EU referendum?
Eligibility will be based on the criteria for voting in a general election, which means citizens of most EU countries (who can vote in local and European elections in Britain) will not be allowed to take part. Anyone over the age of 18 who falls into one of the following groups can cast a vote:
British citizens resident in the UK
British citizens resident overseas for less than 15 years
Citizens of Ireland, Malta and Cyprus resident in the UK
Commonwealth citizens resident in the UK
Commonwealth citizens resident in Gibraltar
However, citizens of Jersey, Guernsey and the Isle of Man, which are not in the EU, will not take part. Members of the House of Lords will be allowed to vote, despite being ineligible to cast a ballot at general elections.
The High Court recently quashed a legal challenge to the rule that prohibits Britons who have lived abroad for more than 15 years from voting. It means that more than 700,000 ex-pats will not have a say in the referendum. They have vowed to appeal, arguing that the referendum result will have a "very real effect" on their lives.
What would happen if the vote was held tomorrow?
The latest poll of polls have shown that exactly 50 per cent of voters support Brexit and 50 per cent would like to remain in the bloc. Carried out by NatCen Social Research, the comprehensive survey takes into account the average of the six most recent surveys. Individually, however, each poll shows contradictory results, with narrow leads for Remain and Leave depending on the survey.
Can we trust the polls?
After last year's general election campaign – in which the opinion polls were very consistent and, as it turned out, very wrong – many onlookers are approaching EU referendum polls with some scepticism. Doubts about their accuracy have been amplified by sharp differences in the outcomes they have predicted: from a 26-point lead for Remain last December to a nine-point lead for Leave two months later.
The problem, according to polling analyst Matt Singh, is the difficulty in finding a representative sample of voters – and the difference in techniques used by online and telephone pollsters explains much of the variation. "This sampling error is increasing the lead for Leave by about 3 percentage points for online polls and increasing it for Remain in the phone polls by about 5 percentage points," says the BBC.
But YouGov's Stephan Shakespeare says in The Times that his company's online polls have corrected for this problem, and that they now represent a true snapshot of public opinion. Even so, he says, they may not predict the result in June. Describing an Out vote as a "massive irreversible change", he says "the deep human drive to avoid risk is likely to become increasingly powerful as we get closer to a real decision which cannot easily be changed once it is made." The result, he says, is that "whatever the state of the polls today, it is impossible right now to predict the level of change yet to come".
Who is campaigning on either side?
Out: Vote Leave, backed by leading Conservative figures Boris Johnson, Michael Gove and Iain Duncan Smith, has been designated as the official Brexit campaign.
In: Sir Stuart Rose, the previous boss of Marks & Spencer, leads Britain Stronger in Europe, which is backed by all three living former prime ministers John Major, Tony Blair and Gordon Brown.