It consists of just five short, vaguely worded paragraphs, but Article 50 of the European Union's 2007 Lisbon Treaty will decide how Britain leaves - and it is already causing problems.
Has it been used before?
No. The short section buried in the laws that govern the EU has never been used before and was written at a time when the prospect of any member state leaving seemed very unlikely.
What does it say?
The key opening phrase - "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements" - was indeed the first time EU law laid out an exit plan.
What is the problem?
London and its soon-to-be former partners are in dispute about how and when Britain will leave after last week's vote to quit. "It provides few concrete details about how a the withdrawal must be organised," Robert Chaouad, a research Fellow at the Institute for International and Strategic Affairs said.
Has the trigger been pulled?
Article 50 says that "a Member State which decides to withdraw shall notify the European Council (the 28 leaders of the EU member states, led by EU President Donald Tusk) of its intention". But it does not say when this must happen, and that has become the first crucial stumbling block.
What is the British Government's view?
Prime Minister David Cameron said he would resign by October, leaving his successor to begin the talks. "I think it is right that this new prime minister takes the decision about when to trigger Article 50 and start the formal and legal process of leaving the EU," he said. Some Brexit campaigners have long said that Britain should aim to negotiate a comprehensive new relationship with the EU, seeking access to markets without submitting to EU rules or open migration, before binding itself into the two-year timetable that would be fixed for talks if Article 50 is triggered.
What about the EU?
Cameron's EU peers believe notification should be "as soon as possible" to minimise the chaos of a Brexit, and preferably by Cameron himself at an EU summit on Wednesday and Thursday. "It doesn't have to be written. He can just say it." one EU official said. EU officials and leaders fear that a prolonged haggling with London will further increase the risk of a domino effect of nationalist-led demands for exit from other states.
Can the EU force the trigger?
Only the member state concerned can make the notification. It cannot be forced on Britain by Brussels. Jean-Claude Piris of the Delors Institute in Brussels said it was "normal and understandable" Cameron wanted to wait until a successor was in place, and that the rest of the EU "will not put a knife to Britain's throat". He added: "However if it drags on another six months or a year then it will become objectionable and I would understand if the EU became impatient".
What does the UK's 'constitutional requirements' mean?
David Allen Green, who writes about law and policy at the Financial Times, wrote on his blog Jack of Kent: "There is an interesting question as to what 'its own constitutional requirements' means in the case of the UK, which does not have a codified constitution. In my view, it could mean the Prime Minister simply making the notification as an exercise of the prerogative, following the referendum result. Or it could mean a prior parliamentary vote". Questions have been raised as to whether the Scottish Parliament's approval would be required - Scots voted strongly to Remain.
Is there any chance the withdrawal could not be triggered?
There have been calls in Britain for the result to be reviewed or for Parliament to ignore the referendum. Green said: "The fact is that the longer the Article 50 notification is put off, the greater the chance it will never be made at all. This is because the longer the delay, the more likely it will be that events will intervene or excuses will be contrived ... Stalemates can last a long time. And unless there is political will to resolve it, this stalemate will not resolve itself."
Is there a get-out clause in the fact that the referendum is non-binding?
Green wrote: "Other ways of solving the problem created by the referendum result may present themselves: another referendum ... or a general election where EU membership is a manifesto issue, or some other thing. This will not please Leave campaigners, and rightly so. It means the result of the referendum will be effectively ignored. But that was always possible, as it was set up deliberately as a non-binding referendum." An EU official, asked whether Britain could launch the process and then ask to stay, said that was not foreseen by the treaty: "Once you trigger it, you cannot take it back".
What is the time-frame for withdrawal?
Under Article 50, Britain's notification will set the clock ticking on a two-year period of negotiations within which a basic withdrawal agreement should be made. After that "the treaties shall cease to apply to the state in question" - or in layman's terms, Brexit is a reality. The talks can in theory be extended if need be - but only by the unanimous consent of Britain and the other 27 member states. The alternative is a chaotic British exit on the stroke of two years, with lots of loose ends untied. If a state fails to agree to a departure treaty with the others, EU law simply stops applying to it after two years.
What else will need to be negotiated?
Britain and the EU will separately need to negotiate what Article 50 calls their "future relationship". This is not spelled out but would include issues such as access to the single market, whether Britain will have trade deals with the EU, whether free movement will continue, and so on. Under Article 50, negotiations on these topics will take place in parallel with the talks on the basic withdrawal agreement.
Is that clear enough?
It is vague, mentioning only the talks "taking account of the framework for its future relationship with the union". Chaouad says: "Legally, everything is possible, because there is no precedent. One can imagine a withdrawal agreement that would double as an agreement establishing relations between the UK and the EU".
How is the withdrawal approved?
The remaining 27 EU states must approve Britain's withdrawal agreement by a "qualified majority". The European Parliament will also vote by a simple majority.
Is there any chance Britain could rejoin?
Article 50 does say that a former EU nation can seek to rejoin after leaving - under Article 49 of the Lisbon Treaty. This however begins the membership process from zero, in the same way as current candidate states like Turkey, Serbia and Albania.
- AFP, Reuters