Article 50: What happens after Brexit process is officially triggered?

Theresa May is due to trigger the formal process to leave the European Union by sending a letter to Donald Tusk invoking Article 50 of the Lisbon Treaty.

The Prime Minister has signed the six-page letter that kicks off two years of negotiations between the UK and the other 27 member countries which will determine what relationship they have after it leaves in 2019.

The mechanism is due to be invoked at 12.30pm UK time when the document is hand-delivered to Mr Tusk, the President of the European Council, and Ms May will then make a formal statement to MPs confirming the start of the process.

The full text of the letter will then be released and copies will be sent to all 27 member states.

Following the start of the official process, the other EU leaders will hold an emergency summit to discuss their joint strategy during the exit talks.

The UK and the EU will spend the next two years hashing out the "divorce agreement" unpicking decades of close political and economic co-operation between the states while trying to negotiate a free trade deal.

Britain will remain in the EU with all the same rights and responsibilities that entails until a withdrawal agreement is reached or when two years have elapsed from the notification of intent to withdraw.

The two-year deadline can be extended, but only if the 27 other EU leaders unanimously agree to it.

Then it is up to the UK and EU to battle it out over terms. The UK is facing an estimated £48bn Brexit "divorce bill" to pay for the contributions it agreed to make to EU-wide projects up to 2020, as well as the pensions of officials.

Meanwhile hardliners have called on Ms May to refuse to pay the bill, leaving her very little room to manoeuvre as she currently holds a narrow majority in the House of Commons and the majority of Tory MPs are pro-Brexit.

The negotiations could also be complicated by the domestic political situation in the individual member countries. Several major EU economies,such as France and Germany, are facing major elections where right-wing populists could make major gains.

This means EU politicians will not want to appear "soft" on the UK for fear that populists in these countries will start advocating for their own withdrawal.

No one is entirely sure.

Ms May has said she will be seeking a "hard Brexit" which means the UK will leave the single market and end freedom of movement for EU citizens.

But the EU's chief Brexit negotiator Guy Verhofstadt has indicated the other member states would not be willing give up on one of the core principles of the union.

If the UK and the EU cannot reached a withdrawal agreement by 29 March 2019, the UK will crash out of the EU and resort to World Trade Organisation (WTO) terms for trade.

This means UK businesses would be paying an average of 14.4 per cent tariffs on agriculture and 4.3 per cent on non-agricultural goods.

As 44 per cent of all UK exports in goods and services went to the EU in 2015, £220bn out of £510bn total exports, this could have a devastating impact on the UK economy.

A leaked Treasury report on the post-Brexit economy suggested Britain could face a "major economic shock" if it leaves the EU without a deal in place and would have less access to the single market than Pakistan, Rwanda or Yemen.

The report described WTO rules on services, which make up almost 80 per cent of the UK economy, as "out of date, based on a set of commitments that are 20 years old".

Despite this several UK politicians, including Foreign Secretary Boris Johnson who said WTO rules would be "perfectly OK" for the UK, have dismissed concerns about the Brexit process as "Project Fear" and attempting to "talk down Britain".

Although the vote to leave the EU happened in June 2016 it was only a signal of the wishes of the UK that they wanted their government to withdraw. It was not legally binding nor did it trigger any formal process.

Ms May said in October that she planned to trigger Article 50 by the end of March because the Government needed time to make preparations for talk.

The government has had to employ a raft of negotiators for the deal – as it has not had to make trade deals for 40 years – and decide what type of Brexit it wants.

Her plans were also derailed somewhat by a successful legal challenge which forced the Government to get the permission of Parliament before triggering Article 50.

It was temporarily frustrated by the House of Lords which attempted to add amendments to the bill which would protect the rights of EU citizens already living in the UK.

No one is entirely sure as the mechanism has never been used before.

Lawyers on both sides in the legal battle through the courts said once Article 50 had been invoked it was irrevocable.

But Lord Kerr, the diplomat who drafted the measure, has suggested a country could change its mind.

Mr Tusk has also indicated that abandoning Brexit could be an option, claiming that other EU leaders would be sympathetic and "if we have a chance to reverse this negative process, we will find allies".

Additional reporting by PA