Britain triggered Article 50 of the EU's Lisbon Treaty, starting the two-year countdown to leaving the bloc.
Just days after the EU's 60th birthday, Britain became the first country ever to seek a divorce, striking a blow at the heart of the union forged from the ashes of World War II.
"This is an historic moment from which there can be no turning back," Prime Minister Theresa May told MPs.
Nine months after the shock referendum vote for Brexit, Britain triggered Article 50 of the EU's Lisbon Treaty, starting the two-year countdown to leaving the bloc.
"We already miss you," said EU President Donald Tusk in Brussels, after receiving the letter formally notifying him of Britain's intention to leave.
The six-page letter was signed by May and delivered in person by British ambassador Tim Barrow in Brussels.
It sets the stage for months of protracted and difficult negotiations between London and Brussels over outstanding bills, immigration and future trade ties.
"We must not forget that the UK is still a partner, in NATO and in Europe," said a spokeswoman for German Chancellor Angela Merkel.
She added that the Article 50 letter would give "more clarity" on Britain's strategy, and "on this basis, the 27 (other) member states and EU institutions will define their interests and aims".
While the EU scrambles to contain the fallout from Britain's departure after four decades of membership, May is also battling to keep her deeply divided nation together.
Scotland's parliament on Tuesday voted to call a fresh referendum on independence from Britain in a bid to hold on to EU ties.
"As we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can -- and must -- bring us together," May told MPs.
In her letter, May repeated 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".
But as with many divorces, negotiations could rapidly turn nasty over money.
The priority is settling Britain's outstanding bills, estimated at between 55 and 60 billion euros ($59-65 billion) -- an early battle that could set the tone for the rest of the talks.
It could be weeks before formal talks start, as the leaders of the other 27 EU nations will meet on April 29 to forge their response.
Both sides also want to resolve the post-Brexit status of more than three million European nationals living in Britain, and one million British expats in the EU.
Forging a new trade agreement and tensions in Northern Ireland -- which will have the UK's only hard border with the EU -- will also cause major headaches.
Many business leaders are deeply uneasy about May's decision to leave Europe's single market, a free trade area of 500 million people, fearing its impact on jobs and economic growth.
The Brexit vote sent the pound plunging, although the economy has been largely stable since then.
Despite May's call for unity, Britons appear as divided now as in June's referendum, which the "Leave" camp won by a narrow 52-48 margin after a vitriolic campaign.
Tens of thousands marched through London on Saturday demanding Britain keep its 44-year-old EU membership, with one banner urging politicians to "stop this madness".
But many were elated after waiting years for this moment, including 66-year-old pensioner Christine Garrett, shopping at an east London street market.
"We could stand on our own two feet as a country. What do they do for us? Nothing," she said.
Pushing her pram nearby was Julia Rogers, 38, who disagreed, saying: "It's going to be a disaster".
In the City of London financial hub, some were worried about the implications of Brexit.
"It's quite a sorry state of affairs," said Daniel Smith, 41.
The famously partisan British press reflected this division as the historic day dawned.
The fiercely eurosceptic Sun beamed "Dover and Out" and "See EU Later" on the White Cliffs of Dover, Britain's closest point to the continent.
On the other side of the divide, the left-leaning Guardian mocked up an EU jigsaw with Britain missing and the headline: "Today Britain steps into the unknown."
The EU is determined to preserve its unity and has said that any Brexit deal must not encourage other countries to follow Britain out of the door.
As she begins Brexit, May is also battling to keep the United Kingdom together and has rebuffed the Scottish parliament's call for a second independence referendum.
Scots overwhelmingly voted for Britain to remain in the EU and are particularly worried about leaving Europe's single market -- the price of controlling immigration.
"Today, the PM will take the UK over a cliff with no idea of the landing place. Scotland didn't vote for it and our voice has been ignored," First Minister Nicola Sturgeon said.
With the challenges ahead, May has said that "no deal is better than a bad deal" and analysts say that threatening to walk away may be her only trump card in a process in which the EU will hold most of the cards.