LONDON (REUTERS, AFP) - Britain will file formal divorce papers to exit the European Union on Wednesday (March 29), 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 is poised to 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 after the shock British referendum vote to leave the EU, British Prime Minister Theresa May will notify EU Council President Donald Tusk in a letter that Britain really is quitting the bloc it joined in 1973.

Mrs May, an initial opponent of Brexit who won the top job in the political turmoil that followed the referendum vote, will then have two years to settle the terms of the divorce before it comes into effect in late March 2019.

"Now that the decision has been made to leave the EU, it is time to come together," Mrs May will tell lawmakers, according to comments supplied by her office.

"We must no longer be defined by the votes we cast in the referendum - but a determination to make a success of the result," she 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.

"When I sit around the negotiating table in the months ahead, I will represent every person in the whole United Kingdom - young and old, rich and poor, city, town, country and all the villages and hamlets in between."

LETTER SET TO BE DELIVERED

Mrs May's notice of the UK's intention to leave the bloc under Article 50 of the EU's Lisbon Treaty is due to be hand-delivered to Mr Tusk in Brussels by Tim Barrow, Britain's permanent representative to the EU.

She has already signed the letter and the two leaders spoke by phone on Tuesday ahead of the momentous event. The Brexit letter is expected to seek to set a positive tone for the talks and recap 12 key points which Mrs May set out as her goals in a speech on Jan 17, EU officials said.

Mr Barrow arrived at the European Council building shortly before 0800 GMT (4pm Singapore) for a routine weekly meeting with the senior diplomats of other member states. He arrived in the ambassadorial Jaguar, carrying a well-worn black leather briefcase which may - or may not - have contained Mrs May's letter. British officials declined to confirm.

Mr Barrow has an appointment with Mr Tusk, the EU summit chair and former Polish prime minister, in the Council President's offices on the top 11th floor of the new Europa Building at 1120 GMT (7.20pm), where he is due to hand over the letter. That moment will formally set the clock ticking on Britain's two-year exit process. Mr Tusk will speak to reporters after that.

Within 48 hours of reading the letter, Mr Tusk will send the 27 other states draft negotiating guidelines. He will outline his views in Malta, where from Wednesday he will be attending a congress of centre-right leaders. Ambassadors of the 27 states will then meet in Brussels to discuss Mr Tusk's draft.

A draft document just hours before London's formal notification to quit seen by Reuters showed that EU leaders will pledge to stand united in "constructive" talks with Britain to reduce uncertainty for citizens and businesses.

Brussels and London face monumentally difficult negotiations over outstanding bills, immigration and future trade ties.

The outcome of the negotiations will shape the future of Britain's US$2.6 trillion (S$3.6 trillion) economy, the world's fifth biggest, and determine whether London can keep its place as one of the top two global financial centres.

For the EU, already reeling from successive crises over debt and refugees, the loss of Britain is the biggest blow yet to 60 years of efforts to forge European unity in the wake of two devastating world wars.

Its leaders say they do not want to punish Britain. But with nationalist, anti-EU parties on the rise across the bloc, they cannot afford to give London generous terms that might encourage other member states to follow its example and break away.

One major uncertainty for May is who will be leading France and Germany, which both face elections this year. "It's bad news for everybody. It's a wedge pushed into the European project," French centrist presidential candidate Emmanuel Macron told Europe 1 radio. The pro-EU Macron has struck a firm line on Brexit, saying he would ensure Britain gains no undue advantages outside the Union.

Meanwhile, the process has already split Britain, where 52 per cent voted for Brexit last June, but 48 per cent wanted to stay in the EU - including a majority in Scotland, which has renewed its threat to secede.

On Tuesday, Scotland's semi-autonomous parliament backed a call by its nationalist government for a new referendum on independence before Brexit.

Scotland is particularly concerned about leaving Europe's single market - the price Mrs May says must be paid to end mass migration, a key voter concern.

The prime minister rebuffed the referendum request and has vowed to fight for a new relationship with Brussels that will leave Britain stronger and more united than before.

BREXIT 'BEST THING'

Britons are as divided as they were in the referendum.

Tens of thousands of people marched through London on Saturday demanding Britain keep its 44-year-old EU membership, urging politicians to "stop this madness".

But many are elated after waiting years for this moment, including 66-year-old pensioner Christine Garrett who was out shopping at a street market in Bethnal Green in east London.

"I think 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.

"I think it's quite a sorry state of affairs," said Mr Daniel Smith, 41, adding: "We've sort of backed ourselves into a corner that we can't get out of".

BUDGET BATTLES

The EU is expected to issue a first response to Britain on Friday, followed by a summit of EU leaders on April 29 to adopt their own guidelines - meaning it could be weeks before formal talks start.

Their priority is settling Britain's outstanding obligations, estimated between 55 and 60 billion euros - an early battle that could set the tone for the rest of the negotiations.

Both sides have also said they are keen to resolve the status of more than three million European nationals living in Britain after Brexit, and one million British expats living in the EU.

The two sides also want to ensure Brexit does not exacerbate tensions in Northern Ireland, the once troubled British province which will become the country's only hard border with the rest of the EU.

Britain's finance minister, Philip Hammond, said on Wednesday he was confident the country would negotiate a customs arrangement with the EU that would allow for borders to be as frictionless as possible after Brexit. "It is not in the interests of anybody on the continent of Europe to have lines of trucks," Mr Hammond said, adding that he did not recognise some of the large numbers being spoken about in Brussels that some officials say Britain may have to pay to the EU as it exits.

Britain also wants to reach a new free trade agreement within the two-year timeframe, although it has conceded that a transitional deal might be necessary to allow Britain to adapt to its new reality.

Many business leaders are deeply uneasy about Mrs 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 economic growth has been largely stable since then.

WALKING AWAY

With the challenges ahead, there is a real chance that negotiations will break down and Britain will be forced out of the EU without any deal in place.

This could be highly damaging for both sides, by erecting trade barriers where none now exist as well as creating huge legal uncertainty.

Mrs May has said that "no deal is better than a bad deal", and she has the support of pro-Brexit hardliners in her Conservative party, who have been campaigning for decades to leave the EU.

While talk of walking away has softened as the negotiations loom, experts say it may be her only trump card in a process in which the ticking clock means the EU will hold most of the cards.

"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.