What is Article 50 of the Lisbon Treaty - and is it irrevocable?

Triggering Article 50 of the Lisbon Treaty will formally begin the UK's withdrawal from the European Union – but it has never before been used and is completely untested.

So what does it actually involve and is there any way back once it has been invoked?

How does Article 50 work?

Article 50 of the Treaty of Lisbon sets out the processes and deadlines that would govern a country leaving the EU – and the UK will be the first member state to use it. 

Any country wishing to exit the block must enter into negotiations with the rest of the EU about the terms of its departure. This can take up to two years, with the possibility of an extension if the withdrawing state and the European Council mutually consent to a longer negotiation period. Some experts warn it could take up to ten years to wrap up the complicated "divorce".

Under the treaty's terms, states are allowed to apply to rejoin the EU once they have left – but they will have to go through the same process as a new applicant. Consequently, the UK would almost certainly have to accept some of the EU principles it has so far avoided, including adopting the euro as currency.

What will it involve?

Talks will focus on negotiating access to the single market and creating new trade deals, as well as deciding the rights of movement for EU nationals and UK citizens. Any trade agreement would have to be approved by the 27 other member states and may require ratification by national parliaments. 

Damian Chalmers, the professor of European law at the London School of Economics, says this will be "difficult" because each state will have its own political imperatives.

How long will it take?

That depends on attitudes across Europe, as well as the political situation at home. "Brexiters, including Ukip's Nigel Farage, tend to play down the complexity of this process; but some experts believe that in reality it could take much more than two years," The Guardian says.

EU leaders are unlikely to be very accommodating to Britain's wishes, says European Policy Centre analyst Fabian Zuleeg. "It is not in [their] political interest to concede a lot to the UK," he told the EU Observer. 

Prime Minister Theresa May has promised to trigger Article 50 by the end of March 2017, although the government is currently facing a legal challenge on whether it must first consult MPs on this.

What will happen to the UK during negotiations?

Some things would continue as normal during the negotiation period. EU laws would still apply to the UK and ministers would continue to participate in most EU business, says Open Europe's co-director Raoul Ruparel. However, the country "would not participate in internal EU discussions or decisions on its own withdrawal".

Can the UK back out once Article 50 is triggered?

Lord Kerr of Kinlochard, who wrote Article 50, says the UK could choose to stay in the EU even after exit negotiations have begun.

"[Article 50] is not irrevocable. You can change your mind while the process is going on. During that period, if a country were to decide, 'Actually we don't want to leave after all,' everybody would be very cross about it being a waste of time, they might try to extract a political price, but legally they couldn't insist that you leave," he says.

The Scottish cross-bench peer believes the country "might want to think again" when the terms of Brexit become clearer. 

Lord Kerr says he never envisaged the UK would make use of Article 50: "I thought the circumstances in which it would be used, if ever, would be when there was a coup in a member state and the EU suspended that country's membership."