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 could it be delayed in the House of Lords?
How does Article 50 work?
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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 bloc 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 the negotiations involve?
Talks will focus on the UK's 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 their national parliaments.
The UK government has assured its own parliamentarians that they will be able to vote on whether or not to accept the final trade deal with the EU. However, even if MPs reject the agreement, Prime Minister Theresa May has said the UK will walk away from the bloc, with no deal in place.
What happens to the UK's place in the EU during negotiations?
Most 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.
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."
Will the House of Lords delay the process?
MPs have already backed a draft law authorising May to trigger Article 50 and peers look unlikely to scupper her plan to do so by the end of March.
The government does not have a majority in the upper chamber, but opposition leader Lady Smith told BBC Radio 4's Today programme: "All the House of Lords can do is ask the Commons to look at an issue again that is built into the government's own timetable. I don't see any extended ping-pong on this at all, I'll be very clear about that."
A coalition of Labour, Liberal Democrat, crossbench and some Conservative peers are, however, planning to force changes to the bill, says The Guardian. They will seek amendments on areas such as the status of EU nationals living in the UK and on securing parliament a "meaningful vote" on the final deal.
Peers are due to begin debating the issues today, although they will not take any votes until next week.
"No 10 appears to be braced to reject any amendments passed by the Lords, with a government source briefing earlier this month that peers who try to hold up the legislation would increase the appetite for reform of the upper chamber," says the Guardian.
