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, with talks probably beginning in earnest in May or June.
Speaking during a visit to Swansea, 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 and all parts of the UK when we enter these negotiations.
"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.
"We are going to be out there, negotiating hard, delivering on what the British people voted for."
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.
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.
The UK will have "the choice to eat what's on the table or not come to the table at all", Mr Juncker told German newspaper Bild am Sonntag.
He predicted that Britain's experience of withdrawal will bring the other 27 member states closer together, as they "see from the UK's example that leaving the EU is a bad idea (and) fall in love with each other again and renew their vows with the European Union".
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.
Ukip leader Paul Nuttall described the announcement as "long overdue", adding: "It's late and we don't want to wait. Now let's crack on with it and stop procrastinating. It's time for Britain to go global."
Labour MP Pat McFadden, a leading supporter of the Open Britain group, said: "As she enters talks with our European partners, it is up to the Prime Minister to deliver the deal that she and her ministers have promised. That means a trade agreement that gives us the 'exact same benefits' as we have now on access to the European market.
"The Government has promised a Brexit deal that will not damage our economy and put jobs at risk. They need to meet the tests they have set themselves."
The Scottish Government complained it had not been notified of the announcement in advance, with minister Michael Russell saying he had learnt about it from the media.
Mrs May is due to visit all four nations of the UK before triggering Article 50. She will address MPs in a statement to the House of Commons following her regular weekly session of Prime Minister's Questions on March 29.
She was cleared to take the step when the European Union (Notification of Withdrawal) Act gained royal assent last week, after a Supreme Court ruling forced her to seek the approval of both Houses of Parliament.
Labour leader Jeremy Corbyn said: "The referendum delivered a clear result to leave the European Union and the Government has a mandate to trigger Article 50, which will begin that process.
"Labour respects the will of the people, but the Government has failed to build a consensus about what form Brexit should take. It has failed to provide clarity about its intentions and it has been reckless about the costs of leaving without securing a deal.
"The Government has no mandate to use Brexit to turn Britain into a bargain basement tax haven. We will hold the Government to account every step of the way and fight to secure jobs and the economy, workers' rights and environmental and consumer protection."
Shadow Brexit secretary Sir Keir Starmer said the triggering of Article 50 was "a hugely significant moment for the whole country" and said it was "extraordinary" that Mrs May had "failed to provide any certainty about her plans for Brexit or to prepare for the clear dangers of not reaching a deal with the EU".
Sir Keir later said Labour was prepared to oppose the final divorce deal.
He said: "If the Prime Minister does not come back with the right deal or even comes back with no deal, then of course we would consider voting against it."
Responding to Mr Juncker's comments, Mrs May's official spokesman said: "We have said many times we are at the start of negotiations and let's see how it unfolds."
Mr Barnier indicated that he was preparing for the possibility that Brexit will mean tighter controls at borders with the UK.
As he held a technical seminar with member states to discuss the impact on customs controls and procedures, the Commission negotiator said: "EU27 have to start preparing now for future controls."
Mrs May's spokesman responded: "The PM has made clear that our aim in this negotiation is to secure frictionless borders and that's what we are seeking to achieve."
SNP Europe spokesman Stephen Gethins said that the Article 50 announcement "shatters beyond repair any notion or position that the Prime Minister is seeking a UK-wide agreement".
Mr Gethins said: "For nine months since the EU referendum, there has been no attempt by the UK Government to seek a meaningful discussion or agreement with the devolved administrations.
"Scotland is at a hugely important crossroads and we cannot be in a position where we drift for the next two years without as much as a clue as to the impact leaving the EU will have.
"That is why it is more crucial than ever that the Scottish Parliament is able to give the people of Scotland a fair, legal and informed choice on the direction this country takes."