The Conservative election victory earlier this year activated a manifesto pledge to hold an in/out referendum on Britain's membership of the EU by the end of 2017.
David Cameron promised to deliver the vote 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.
The prime minister has since embarked on a campaign to renegotiate the UK's relationship with the union ahead of the referendum and has outlined four key areas where he is seeking reform.
He has vowed to campaign with his "heart and soul" to keep Britain inside a reformed EU if an agreement on these reforms is reached.
But a reluctance from within the EU for such changes – particularly those which require treaty change – could thwart Cameron's plans.
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
Perhaps 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.
Trade: One of the biggest advantages of the EU is free trade between member nations, making it easier and cheaper for British companies to export their goods to Europe. Some business leaders think these savings outweigh the billions of pounds in membership fees Britain would save if it left the EU. The UK also risks losing some of its negotiation power internationally by leaving the trading bloc, but it would be free to establish trade agreements with non-EU countries.
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. The Economist says Britain would still be subject to the politics and economics of Europe, but would no longer have a seat at the table to try to influence matters.
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. However, it says that GDP could rise by 1.6 per cent if the UK could negotiate a free trade deal with Europe and pursued "very ambitious deregulation".
Jobs: Free movement of people across the EU opens up job opportunities for UK workers willing to travel and makes it relatively easy for UK companies to employ workers from other EU countries. Ukip says this prevents the UK "managing its own borders". But, writing for the LSE, Professor Adrian Favell says limiting this freedom would deter the "brightest and the best" of the continent from coming to Britain, create complex new immigration controls and reduce the pool of candidates employers can choose from.
Regulations: 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. However, others warn that millions of jobs could be lost if global manufacturers, such as car makers, move to lower-cost EU countries, while British farmers would lose billions in EU subsidies.
Influence: Britain may lose some of its military influence – many believe that America would consider Britain to be a less useful ally if it was detached from Europe.
On the plus side, The Economist says Britain would also be able to claim back its territorial fishing waters, scrap caps on limits to the number of hours people can work per week, free itself from the EU's renewable energy drive and create a freer economic market. This would turn London into a "freewheeling hub for emerging-market finance – a sort of Singapore on steroids", it says.
But it concludes that the most likely outcome is that Britain would find itself "as a scratchy outsider with somewhat limited access to the single market, almost no influence and few friends. And one certainty: that having once departed, it would be all but impossible to get back in again."
EU referendum: the big questions for Britain
When will the EU referendum be held?
The date is not yet known, although Cameron has committed to holding it before the end of 2017. Some have suggested that the pace with which he has begun negotiations suggests the vote could be called next year in order to avoid a long period of uncertainty that might hurt businesses.
The government has ruled out holding it on 5 May 2016, the same day as elections to devolved parliaments in Scotland, Wales and Northern Ireland and for London's mayor. But this does not mean an early referendum is off the table.
What will the referendum ask?
The Tories recommended that the question 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.
What would happen if the vote was held tomorrow?
Britons narrowly favour leaving the EU, according to the latest opinion polls. The YouGov survey revealed that 40 per cent of people would vote in favour of a "Brexit", 38 per cent supported continued membership, 16 per cent were undecided and six per cent would not vote.
Who is campaigning on either side?
Several 'In' and 'Out' groups have launched in recent weeks and both sides have already begun campaigning hard. The main campaign trying to convince voters that the UK should remain in the EU is Britain Stronger in Europe led by the Tory peer and former M&S boss Lord Stuart Rose.
On the Eurosceptic side, Vote Leave – which has the backing of some big-name business leaders – is confident it will beat Nigel Farage's Leave.EU to become the official group campaigning for a Brexit. The group appointed as the official campaign on either side will have the additional benefits of higher spending limits, television broadcasts and a grant.
What does British business say?
Most business leaders are thought to be strongly in favour of staying in the EU, but all the early running has been made by a business-focused anti-EU group. Business for Britain is led by the former chief executive of the right-leaning think-tank, the Taxpayers' Alliance, and funded by the Daily Telegraph. Its supporters include the director general of the Institute of Economic Affairs, Mark Littlewood, and the chief executive of fund management business Newton, Helena Morrissey CBE, John Caudwell, founder of Phones4u, Peter Goldstein, co-founder of Superdrug, Sir Stuart Rose, former chairman of Marks and Spencer, and Next chief executive Lord Wolfson.
What about the rest of the business community?
As a whole, entrepreneurs and business owners in Britain broadly remain in favour of remaining a member - and their enthusiasm may be growing. According to the Financial Times a survey of 3,800 businesses earlier this year found 63 per cent believe that leaving the EU would have a negative impact for Britain, a rise of 4 percentage points since the end of 2014. The sentiment seems to be reciprocated: of 2,600 senior executives in 36 countries across Europe in February, nearly two-thirds of those in the eurozone said a British exit from the EU would have a negative impact on the European economy.
How important is the EU to the UK in trade?
The EU is our biggest trading partner and the largest single consumer market in the world, and thus to leave it would certainly involve risk. According to official figures, Britain exported about £147bn worth of goods and services to EU countries last year, a fall of about two per cent from 2013. As a block this makes it our biggest partner, but the top five individual nations include only two EU members: Germany and France. Anti-EU business leaders argue we would do better with China, currently our third biggest trading partner, if we were able to negotiate our own deals outside the EU. A secondary issue involves countries outside Europe which work with the UK because of its access to the European market. Barack Obama has repeatedly expressed a preference for the UK remaining in the UK, and ratings agency Standard & Poor's has suggested that banking groups might take their headquarters elsewhere if Britain were to leave the EU.