Theresa May is poised to finally trigger Article 50 tomorrow and start the official process of Brexit.

The Prime Minister has written a letter that will be delivered by hand to EU Council President Donald Tusk at around 12.30pm.

This will be the moment in history as Article 50 is not triggered until notification is received in Brussels.

The letter itself - rumoured to run to eight pages - will be signed by the Prime Minister later tonight.

The document was said by No 10 to be one of the most important in Britain's recent history as it officially begins the irreversible Brexit process.

Theresa May, pictured in Birmingham today, is poised to finally trigger Article 50 tomorrow and start the official process of Britain's departure and ensure Brexit cannot be reversed

The Prime Minister will write a letter that will be delivered by hand to EU Council President Donald Tusk, pictured marking the EU's 60th anniversary on Saturday, at around 12.30pm

The choreography of the historic day will begin at 8am, when the Prime Minister will meet her Cabinet in Downing Street to brief her senior team on its contents.

ARTICLE 50: WHAT HAPPENS NEXT Theresa May's letter to Donald Tusk invoking Article 50 will be handed over at around 12.30pm. This is the point of no return and will trigger two years of frantic diplomacy and negotiations. Within 48 hours, the EU will issue its first response and the 27 remaining members are set to meet in late April. The first major summit is likely to be later in the spring - but talks could be limited by pending elections in Germany. The final deal is expected to emerge by the end of next year in time for a series of votes on ratification Brussels, London and around Europe. If the talks collapse at any point in the two years, Britain could face leaving the EU without a deal at all. Advertisement

The occasion will be similar to the Cabinet meeting held hours before every Budget.

While ministers discuss it, the letter - the original copy complete with Mrs May's so-called 'wet signature' - will be couriered to Brussels.

Following the normal Prime Minister's Questions, Mrs May will make a speech to MPs on the delivery of the letter and its contents.

She can expect a barrage of questions from MPs that lasts at least two hours - more if Commons Speaker John Bercow allows it.

As she stands up, shortly after 12.30pm, Britain's EU ambassador Sir Tim Barrow will personally hand the letter to Mr Tusk.

The trigger is irreversible. It will begin a two year process of negotiations that will expire on March 29, 2019.

While a transitional period is thought likely, Britain will cease to be a full member of the EU.

Some 279 days will have passed since the referendum on June 23, 2016 when Article 50 is invoked - and Brexit will occur some 1,009 days after polling day.

Brexit Secretary David Davis and Home Secretary Amber Rudd were both seen arriving in Downing Street tonight as the finishing touches were put to the Article 50 letter

The document was said by No 10 to be one of the most important in Britain's recent history as it officially begins the irreversible two-year Brexit process set to end on March 29, 2019

Mrs May today revealed she plans for the negotiations to include a whole range of security and justice issues as well as the expected divorce and trading arrangements.

She told the Birmingham Mail: 'The important thing is that although a lot of the discussion is about the trade agreement we will be negotiating with the EU, there will be other elements to our negotiations.

'The whole question of co-operation on security and justice and home affairs will be part of that as well.

'There will be issues like the European Arrest Warrant, like access to Europol.

'These are all aspects of that security and justice and home affairs area that will be part of the negotiations which will start once we've triggered [Article 50] and once the EU council have set their guidelines.'

Mrs May, pictured today addressing the Qatar-UK Business Forum, has revealed she plans for the negotiations to include a whole range of security and justice issues

Speaking at the Qatar-UK Business and Investment Forum, Mrs May welcomed continued foreign investment from the Gulf state ahead of the talks.

She said: 'I hope we can pave the way for an ambitious trade arrangement for when the UK has left the EU, including exploring whether we can forge a new trade arrangement for the whole of the Gulf area.

'As a global Britain, I am determined that we will be the most committed and most passionate advocate of free trade in the world.'

After he has received the letter, Mr Tusk is expected to make a short statement acknowledging he has it.

Within 48 hours, the EU will provide an initial response setting out its guidelines for the talks.

This could make clear the first stumbling block if threats about the need to agree the divorce bill before anything else is committed to paper.

The other EU nations are then set to gather to discuss Brexit toward the end of April.