Britain formally launches the process for leaving the European Union on Wednesday, a historic step that has divided the country and thrown into question the future of the European project.
Just days after the EU's 60th birthday, Britain will become 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 since the shock referendum vote to leave the EU, Prime Minister Theresa May will trigger Article 50 of the bloc's Lisbon Treaty, meaning Britain is set to leave in 2019.
"We must no longer be defined by the votes we cast in the referendum but a determination to make a success of the result," May will tell MPs later Wednesday, according to pre-released extracts of her speech.
"The triggering of Article 50 is the moment for the country to come together," May will say, 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.
May has already signed the Brexit letter to be delivered to EU president Donald Tusk on Wednesday and the two leaders spoke by phone ahead of the momentous event.
- 'Monumentally difficult' talks -
After the historic triggering of divorce proceedings, Brussels and London face months of monumentally difficult negotiations over outstanding bills, immigration and future trade ties.
The EU is expected to issue a first response to Britain on Friday, followed by a summit of leaders on April 29 to adopt their own guidelines -- meaning it could be weeks before formal talks start.
As with many divorces, negotiations could rapidly turn nasty over money.
The priority is settling Britain's outstanding obligations, estimated between 55 and 60 billion euros ($59-65 billion) -- an early battle that could set the tone for the rest of the talks.
Both sides also want to resolve the status of more than three million European nationals living in Britain after Brexit, and one million British expats in the EU.
Forging a new trade agreement and tensions in Northern Ireland -- which will become the country's only hard border with the EU -- will also provide 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.
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 are 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, employees were mostly 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", picturing 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."
Even financial analysts were divided.
Berenbank's chief economist Holger Schmieding described it as "the worst setback ever for the process of European integration".
But Michael Hewson from CMC Markets said that comparing today's events to a step into the unknown was "somewhat melodramatic, and the sort of nonsense we heard in the lead up to last year's referendum."
- Deal or no deal? -
The EU is determined to preserve its unity and has said that any Brexit deal must not encourage other countries to follow Britain out 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 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.
"This marks the end of the period when our government was in control," said Professor Anand Menon, director of the "UK in a Changing Europe" programme.