Theresa May Niall Carson PA Wire/PA Images

LONDON – On Wednesday Theresa May will announce to Parliament that Article 50 has been triggered — beginning Britain's two-year withdrawal from the EU.

Here's how it's all likely to play out.

The letter

Article 50 has never been triggered before so nobody really knows how it's meant to work. However, Downing Street sources suggest it will take the form of a letter. The UK's permanent representative to the EU, Tim Barrow, is expected to hand the letter in person to EU Council president Donald Tusk on Wednesday.

Downing Street sources suggest we won't know the precise wording of the letter, or even whether it will be a lengthy missive or a simple two sentence memo, until after it's been handed over.

What's next?

Donald Tusk is then expected to reply to the letter within 48 hours, after which the 18-month negotiation period for Britain's exit from the EU will begin.

What's first on the agenda for negotiations?

EU leaders have previously insisted that they will not begin any discussions of a Brexit trade deal with the UK until the matter of Britain's administrative exit arrangements are agreed. The main issue here is the question of what bill, if any, Britain will pay the EU when it leaves. Britain's former permanent representative to the EU, Ivan Rogers has suggested that the other 27 EU countries will be angling for a bill of up to £60 billion, while a growing number of Conservative backbenchers believe that the UK should not be liable for any divorce bill at all. Reconciling those two wildly opposing expectations is likely to be the toughest early test for May.

Will the UK parliament play a role?

The main role for the UK parliament will be in adjusting Britain's legal framework for when Brexit takes place. The first step to achieving that will be the Great Repeal Bill. This will give the PM the power to repeal the European Communities Act, which is the foundation for most EU law. The bill will give the government the power to take a 'snapshot' of EU laws applying to the UK at the moment Britain leaves. These will then be automatically transferred over to UK law before going through a process of adjustment and repeal.

Yves Herman/Reuters

Will the Great Repeal Bill pass?

The bill risks being hugely controversial as it will hand sweeping powers to ministers to rewrite Britain's entire legal framework at the swipe of a pen, through the use of delegated powers. These powers are necessary because of the huge number of changes that need to be made to UK law before Brexit takes place. Downing Street insist that all significant changes will require primary legislation in parliament, however, opponents believe these powers will be abused by ministers.

Passing the Great Repeal Bill could therefore be tricky. Remain-leaning Conservative MPs are threatening to block the bill if negotiaions are going badly, while Labour is also considering voting against unless May puts in protections to prevent a central goverment power grab.

What happens if it doesn't pass?

Theresa May would be in an extremely difficult position. She would then face the choice of either seeking to delay Brexit, or calling a snap general election in order to get the bill through.

Can Article 50 be revoked?

We don't know for sure. However, Lord Kerr, who wrote Article 50 believes it can be.

"It is not irrevocable," he said last November.

"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."

EU Council president Donald Tusk also suggested last year that Britain could change its mind.

"In my opinion, the only real alternative to a 'hard Brexit' is 'no Brexit'" he said last October, adding that once exit discussions are over Britain must “assess the outcome of the negotiations and determine if Brexit is really in their interest."

The Supreme Court have treated Article 50 as if it were irrevocable but did not make a ruling on the subject.

However, most international organisations allow a “cooling off” period between the notification of withdrawal and the moment that countries leave them. In 1928 Spain famously decided not to withdraw from the League of Nations shortly before their withdrawal was due to take effect.

Regardless of whether they have the power to revoke, Downing Streets insists that they will never do so.