- Britain's permanent representative to the EU, Tim Barrow, has handed notice of the UK's intention to leave the bloc under Article 50 to President of the European Council, Donald Tusk
- Addressing parliament, Prime Minister Theresa May said it was "an historic moment from which there can be no turning back"
- EC President Tusk said the EU would remain determined and united.
All updates in Central European Time (CET):
16.24 Chancellor Angela Merkel said in a speech in Berlin that Germany would push for the impact of Brexit for EU citizens living in Britain to be as minimal as possible. She said the talks must first focus on undoing four decades of ties with the UK.
"Only when these questions are cleared up can we subsequently - but hopefully soon - talk about our future relationship," Merkel said.
15.57 German Chancellor Angela Merkel says neither Germany nor the rest of Europe want Britain to leave but that they respect the British voters' decision to do so. She says Germany will work hard to ensure as little impact as possible for EU citizens living in the UK.
Merkel said the EU would take a "fair and constructive" approach in Brexit talks, hoping the UK would do the same. She said the divorce terms would be agreed to first and then "hopefully soon" talks would start on the future relationship between the UK and the EU.
15.53 French President Francois Hollande predicts a difficult two years but said it could not stretch beyond that period.
"I think it will be painful for the British," he said in Indonesia, believing that the Brexit would "force Europe to move forward, no doubt with different speeds."
"It will end with a trade agreement between the United Kingdom and Europe, hopefully one that this is the best trade agreement possible for Europe and for the United Kingdom."
15.42 The EU Parliament warns that it is against EU law for Britain to start negotiating trade agreements with individual nations before it has fully withdrawn from the bloc, according to a draft resolution seen by the Associated Press news agency.
15.39 Reactions from European leaders are now pouring in. Swedish Prime Minister Stefan Lofven says he wants "to see organized and result-oriented negotiations" with Britain, saying good relations with London are "important for Britain, for Europe and for Sweden."
Latvian Foreign Minister Edgar Rinkevics tweeted "As UK has formally triggered Article 50 we should negotiate in a constructive way to forge a fair deal for both EU and UK."
Luxembourg's Foreign Minister Jean Asselborn says "we cannot punish a country that wants to leave the European Union" but "when Theresa May says no deal is better than a bad deal I think that's for both sides, not only for one side."
Estonian Prime Minister Juri Ratas says that "we cannot allow ourselves to be paralyzed by Brexit" and "we must decisively move on together."
15.26 Drinking sparkling wine is a notably European way of celebrating ... and is that bottle Spanish?
15.25 UKIP, the party that spearheaded the Brexit movement are out celebrating. Above MEP Ray Finch celebrates in Brussels. Former leader Nigel Farage said, "The impossible dream is happening. Today we pass the point of no return."
15.07 French centrist presidential candidate Emmanuel Macron said his priority would be to protect the bloc and European interests but that Britain needed to remain close to the EU.
Speaking after a meeting with London Mayor Sadiq Khan, Macron noted that Khan had 200,000 French citizens living in London and said he would work to determine how Brexit will affect them.
"Some of them will decide to come back, I will be very happy to host them again, but, obviously, we will work together with the UK and we will work together with London because they live there."
14.58 In her letter, Prime Minister May struck a conciliatory tone and called for a new, "bold and ambitious" free trade deal with the EU. She said she wanted a "deep and special partnership" that would comprise both security and economic cooperation.
"Europe's security is more fragile today than at any time since the end of the Cold War," she said.
She also said that both Britain and the EU "would of course cope" if the negotiations fall through and the two sides are forced to fall back on World Trade Organization tariffs to trade with each other but added: "It is not the outcome that either side should seek. We must therefore work hard to avoid that outcome.
Her proposed principles for discussions: "1. We should engage with one another constructively and respectfully, in a spirit of sincere cooperation; 2. We should always put our citizens first; 3. We should work towards securing a comprehensive agreement; 4. We should work together to minimize disruption and give as much certainty as possible; 5. In particular, we must pay attention to the UK's unique relationship with the Republic of Ireland and the importance of the peace process in Northern Ireland; 6. We should begin technical talks on detailed policy areas as soon as possible, but we should prioritize the biggest challenges; 7. We should continue to work together to advance and protect our shared European values."
14.49 More candid comments from Germany, this time from Foreign Ministry spokesman Martin Schäfer in Berlin.
"The time-frame is damn narrow and all of the participants know that, including the Brits," he said.
"There are innumerable issues that need to be clarified to prevent uncertainty on both sides of the English Channel as uncertainty is poison for people, the EU citizens, the Germans who live in Britain and don't know what their future status might be and vice-versa for British citizens in the EU.
"Sometimes you wonder if everyone in London has understood what consequences that has, especially for the British economy."
14.43 The EU Council had this video ready to go, explaining some of the morass of bureaucracy that will follow today's letter.
14.18 The full Brexit letter has been made public. In its six pages, May sets the agenda for Brexit, saying the two sides have to work hard to avoid a failure in negotiations. She writes that the vote for Brexit was not intended to harm the EU, and said she wanted a "new deep and special relationship with a strong European Union".
14.12 Germany's foreign minister said it was difficult to understand how any country could believe it would be better off alone in the current global environment.
"The negotiations will surely not be easy for either side," Sigmar Gabriel said in Berlin.
"Bad feelings are understandable. For many it is difficult to understand, especially in these turbulent times, how anyone can believe they would be better off alone. But this can't be the basis for defining our future relationship."
14.09 The European Council released a statement saying the Council had received the letter and would work to adopt guidelines for negotiations.
"We regret that the United Kingdom will leave the European Union, but we are ready for the process that we now will have to follow."
"In these negotiations the Union will act as one and preserve its interests. Our first priority will be to minimise the uncertainty caused by the decision of the United Kingdom for our citizens, businesses and Member States. Therefore, we will start by focusing on all key arrangements for an orderly withdrawal.
"We will approach these talks constructively and strive to find an agreement. In the future, we hope to have the United Kingdom as a close partner."
14.07 In Brussels, European Council President Donald Tusk has given a conspicuously short statement. "There is no reason to pretend this is a happy day, neither in Britain or in London," he says.
He talks of nearly half of British voters who "wish to stay together rather than drift apart."
"There is nothing to win in this process and I'm talking about both sides. In a sense this is about damage control. Our goal is clear; to minimize the costs for European citizens, businesses and member states. We will do everything in our power and we have all the tools to achieve these goals."
However, he adds: "There is something positive in Brexit. Brexit has made us, the community of 27, more determined and more united than ever before."
14.01 Opposition Labour party leader Jeremy Corbyn has demanded May push for access to the European single market.
"The direction the prime minister is threatening to take this country is is both reckless and damaging," he told parliament.
"It would be a national failure of historic proportions (if we left the EU) without having secured protection for jobs and living standards."
He said he would push the government to maintain an open border in Ireland and protect the rights of EU citizens in the UK.
13:57 Part of the process will be the Great Repeal Bill, which rescinds the 1972 European Communities Act under which Britain joined the EU and which subsumed more than four decades of European law into British law.
13:52 May expresses a "fierce determination to get the right deal for every person in this country," and urges national unity.
"We are going to take control of the things that matter most to us.
"It is time to come together, for this great national moment needs a great national effort."
13:48 May emphasizes the future importance of Britain's friendship with Europe, talking of a "deep and special partnership."
"We will continue to be reliable allies, willing partners and close friends."
13:45 May promises that the UK will work closely with European partners, even after Brexit. "I am ambitious for Britain and the objectives I've set out for these negotiations remain. We will deliver certainty wherever possible." May tells the House of Commons a white paper on aspects of EU and British law will be published on Thursday.
The Prime Minister strikes a unionist tone. "We will strengthen the union of the four nations that comprise our United Kingdom," says May. "We will negotiate as one United Kingdom." The PM promises that some powers returned from Brussels will go to devolved governing bodies in Belfast, Edinburgh and Cardiff.
13:38 "This is a historic moment from which there is no turning back," Theresa May tells parliament, promising to build a "stronger and fairer Britain.
"At moments like these, the choice we make defines the character of our nation we can choose to turn our face to the past and say it can't be done, or we can choose to look forward.
"Leaving the EU presents us with a unique opportunity. It's a chance to step back and ask ourselves what sort of country we want to be."
13:28 President of the European Council Donald Tusk has received Britain's formal notification to invoke Article 50.
The notication came as a letter saying Britain wishes to invoke Article 50 of the Lisbon Treaty, which allows a nation to leave the EU after up to two years of negotiations. It was handed over in Brussels by the UK representative to the EU, Sir Tim Barrow.
13:25 Today is Day One of a very difficult road, says the EU's chief negotiator, Michel Bernier. Earlier, British Finance Minister Philip Hammond said Britain was realistic and had "sent a clear signal to our European partners that we understand that we can't cherry-pick, we can't have our cake and eat it."
13:20 The Scottish National Party's Westminster leader Angus Robertson says May promised to secure UK-wide agreement with Scotland, Wales and Northern Ireland on Brexit before invoking Article 50. Robertson says she hasn't done that and says she has "broken her promise and broken her word".
13:10 The German Foreign Ministry says Britain will stay a "close partner and friend" despite the decision to leave the EU. However, it added that "being a close friend is not the same as being part of the family."
13:00 Prime Minister Theresa May has begun her usual weekly answering of questions posed by members of the House of Commons. One of the early questions concerns Brexit and Northern Ireland. May assures the House that her government is "fully committed to ensuring the unique interests of Northern Ireland are protected and advanced" as the negotiations continue.
12:55 Britain's euroskeptic press has been revelling in the impending beginning of the end of Britain's membership of the EU. The Sun, which is Britain's most popular newspaper, went as far as to beam headlines "SEE EU LATER" and "DOVER AND OUT" onto the iconic White Cliffs of Dover, a landmark often seen as representative of Britain's independence and separateness from the rest of Europe.
12:50 Someone's looking pleased with himself: former UK Independence Party leader Nigel Farage, a longtime campaigner for Brexit, is spotted outside the Houses of Parliament. "At 12:30 we pass the point of no return," says Farage.
12:15 Here's Sir Tim Barrow, Britain's man in Brussels. He arrived in the Belgian capital on Tuesday night aboard a Eurostar train. The briefcase he's seen carrying here is thought to contain the letter that will invoke Article 50. The moment that the document is handed over will mark the beginning of the exit process.
12:05 British Prime Minister Theresa May, who campaigned in favor of Britain remaining in the EU, is now on her way to parliament to address members about the Brexit process. Last night she signed the letter that is about to be handed over.
11:45 Pro-EU supporters are gathered outside the Houses of Parliament to make their feelings clear.
11:15 The EU blocks the blockbuster merger of the London Stock Exchange with Germany's Deutsche Börse amid concerns and the fallout likely to arise from Brexit. While not directly related to what is happening in Brussels, the timing is certainly interesting.