London - Britain formally launched the process for leaving the European Union on Wednesday, a historic move that has split the country and thrown into question the future of the European project.
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.
Nine months after the shock referendum vote to leave the bloc, Britain handed over a momentous letter to the EU president in Brussels, triggering Article 50 of the bloc's Lisbon Treaty and firing the starting gun on a two-year countdown towards Brexit.
The notification kickstarts months of what will be protracted and difficult negotiations between London and Brussels over outstanding bills, immigration and future trade ties.
Tim Barrow, Britain's ambassador to the European Union, personally delivered the letter to EU President Donald Tusk's office in Brussels, an AFP journalist said.
Finance minister Philip Hammond said the letter would set out "how we want to take the negotiation forward and how we see this negotiation developing".
While the EU scrambles to contain the fallout from Britain's departure after four decades of membership, British Prime Minister Theresa May is also battling to keep her deeply divided nation together.
The Article 50 trigger comes a day after Scotland's parliament voted in favour of holding a fresh referendum on independence from Britain in a bid to hold on to EU ties.
Monumentally difficult talks
Leaders of the other 27 EU nations are holding a summit on April 29 to forge their own response to Brexit, and it could be weeks before formal talks start.
"The die is cast!" European Parliament president Antonio Tajani told the German newspaper Die Welt.
"In the end, we have to be clear that being a member state of the EU is different from being a partner." 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 (S$82-90 billion) - an early battle that could set the tone for the rest of the talks.
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.
'Stop this madness'
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.
'Dover and Out'
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.
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.
Nevertheless, if talks break down and there is no agreement, it would be highly damaging for both sides by erecting trade barriers where none now exist as well as creating huge legal uncertainty.
Terry Scuoler, head of Britain's manufacturing association EEF, warned: "Forget all notion of crashing out of the EU without a deal and leaving business to pick up the pieces.
"A chaotic Brexit that results in the UK falling back on WTO rules will benefit no one," he said.