Theresa May has promised to “negotiate hard” for the best possible deal for Britain as it leaves the European Union, as it was announced that she will trigger the beginning of formal withdrawal talks on March 29.

The Prime Minister’s letter officially notifying the European Council of the UK’s intention to quit under Article 50 of the EU treaties will set in train a two-year negotiation process expected to lead to Britain leaving the EU on March 29 2019.

Brexit Secretary David Davis said the move would initiate “the most important negotiation for this country for a generation”, with the Government aiming to secure “a new, positive partnership between the UK and our friends and allies in the European Union”.

But sterling dipped on the news, giving up gains and slumping against the dollar to 1.23. The pound also dived against the euro, trading 0.1% down at 1.15.

European Council president Donald Tusk confirmed that he will present draft Brexit guidelines to the remaining 27 member states within 48 hours of notification.

The EU27 are then expected to stage an extraordinary summit within four to six weeks to agree a mandate for European Commission chief negotiator Michel Barnier.

Mrs May said: “I am very clear that I want to ensure we get the best possible deal for the United Kingdom that works for everyone across the United Kingdom.

“I have set out my objectives. These include getting a good free trade deal. They include putting issues like continuing working together on issues like security at the core of what we are doing.”

Britain’s ambassador to the EU, Sir Tim Barrow, informed Mr Tusk’s office on Monday morning of the Prime Minister’s plans, which allow her to meet her self-imposed deadline of the end of March to get the withdrawal process under way.

Notification comes 279 days after the referendum of June 23 last year delivered a 52%-48% majority in favour of withdrawal and five days after the other 27 members gather in Rome to celebrate the EU’s 60th anniversary.

It will be the first time that the provisions of Article 50 – which sets out the process for any EU member state “to withdraw from the Union in accordance with its own constitutional requirements” – have been activated.

Theresa May will trigger Article 50 on March 29, what happens next? (PA Graphics)
Theresa May will trigger Article 50 on March 29, what happens next? (PA Graphics)

Mrs May’s official spokesman said Britain wants to start withdrawal negotiations “promptly”, but accepts that “it is right that the 27 have a chance to agree their position” before talks start.

The announcement came shortly after Commission president Jean-Claude Juncker warned that Britain may have to abandon its hopes of a trade deal if it rejects the terms offered by the EU – which are widely expected to include a “divorce bill” of as much as £50 billion.

Liberal Democrat leader Tim Farron accused Mrs May of rushing into negotiations “without a plan, and without a clue”.

The “hard Brexit” vision she has outlined would “unleash division and bitterness” and should be subject to a second referendum, he said.