Article 50 triggered: the reaction in Parliament
The letter that was delivered to European Council President Donald Tusk, giving formal notice of Brexit, marked the beginning of two years of talks.
At the same time as the letter was delivered, Theresa May came to the House of Commons to make a statement to MPs on the triggering of Article 50 - and the UK's departure from the EU.
Mrs May called it a "historic moment" and said "there can be no turning back".
From Brussels, Mr Tusk said: "We already miss you. Thank you and goodbye."
The prime minister told MPs the government "will ensure that workers' rights are fully protected and maintained" and that "we will pursue a bold and ambitious free trade agreement with the European Union that allows for the freest possible trade in goods and services between Britain and the EU's member states".
Acknowledging it was "a day of celebration for some and disappointment for others", Ms May concluded by appealing for unity and for building a Britain "our children and grandchildren are proud to call home".
As expected, MPs from all sides wanted to have their say and the prime minister was on her feet for almost three hours answering questions from 113 backbenchers.
Here are a few responses to the statement:
Jeremy Corbyn, Leader of the Opposition
"The British people made the decision to leave the European Union and Labour respects that decision. The next steps along this journey are the most crucial.
"…It would be a national failure of historic proportions if the prime minister came back from Brussels without having secured protection for jobs and living standards, so we will use every parliamentary opportunity to ensure the government are held to account at every stage of the negotiations."
Angus Robertson, SNP Westminster Leader
"It is important for everybody to remember on this day that in the referendum on the European Union, the people of Scotland voted by 62% to remain in the European Union. Every single local government area in the country voted to remain in the European Union."
Tim Farron, Liberal Democrat Leader
"Our children and grandchildren will judge all of us for our actions during these times. I am determined to be able to look my children in the eye and say that I did everything to prevent this calamity that the prime minister has today chosen."
Anna Soubry, Conservative
"The prime minister has made it very clear that immigration is her No.1 priority, and that as a result we cannot accept the free movement of people and therefore we cannot remain a member of the single market. But that may change in the next two years."
Hilary Benn, Chair, Exiting the EU committee
"…How will the prime minister ensure there is sufficient time to reach the agreement to provide tariff and barrier-free trade and access to the European market for our services that she has promised Britain's businesses she will bring back from the negotiations?"
Jacob Rees-Mogg, Conservative
"I wish the prime minister good luck and good fortune in her negotiations until she comes to true glory and is welcomed back to this House as a 21st century Gloriana."
Ed Miliband, Labour
"The prime minister is right to say in her statement that the eventual deal must work for the 48% as well as the 52%, because whether we were remainers or leavers, we will live in the same country together after Brexit."
Dame Margaret Beckett, Labour
"There are many across this House who will be very aware of the sheer scale and complexity of the negotiations that will face our team, and very conscious of the importance of getting those right. It has never been more true that the devil will be in the detail."
Joanna Cherry, SNP
"After the Brexit deal has been negotiated, the European Parliament and every other member state in the European Union will have a say on whether to accept that deal. Can the prime minister not see that to deny the people of Scotland a say at the same time would show utter contempt for democracy in Scotland?"
Sir Bill Cash, Conservative
"Can the prime minister reaffirm that at the very heart of this letter lies the democratic decision of the referendum of UK voters given to them by a sovereign Act of Parliament by six to one in this House, enabling the British people to regain their birthright to govern themselves for which people fought and died over generations?"
Kate Hoey, Labour
"On what is a genuinely historic day for our country, may I pay tribute to the prime minister and to the Brexit ministers for their determination and dedication in getting to this stage today to implement the will of the British people."
Alastair Burt, Conservative
"Like millions of others in the United Kingdom, I am proud of the European Union and the contribution that the UK has made to it during my political lifetime, and I am a little sad about today. However, I stand unequivocally with the prime minister as she calls for a united approach to a new future."
Margaret Ritchie, SDLP
"Will the prime minister outline how she will ensure the protection of our fragile economy in Northern Ireland, and ensure tariff-free and continued access to the single European market, which is vital to the growing economy of the island of Ireland?"