By Guy Faulconbridge and Kylie MacLellan

LONDON (Reuters) - The United Kingdom leaves the European Union on Friday, its most significant change of course since the loss of its empire - and a major blow to 70 years of efforts to forge European unity from the ruins of two world wars.

As the EU's most reluctant member prepared to cast off an hour before midnight, Brussels warned that leaving would always be worse than staying, while Britons either side of the Brexit divide expressed either sadness or delight.

After the numerous twists and turns of a 3-1/2-year crisis, the final parting is an anticlimax of sorts: Britain steps into the twilight zone of a transition period that preserves membership in all but name until the end of this year.

At a stroke, the EU will lose 15% of its economy, its biggest military spender and the world's international financial capital - London.

Prime Minister Boris Johnson planned to celebrate with English sparkling wine and a distinctly British array of canapés including Shropshire blue cheese and Yorkshire puddings with beef and horseradish.

"This is the moment when the dawn breaks and the curtain goes up on a new act," said Johnson, one of the leaders of the "Leave" campaign in the 2016 referendum. "It is a moment of real national renewal and change."

Beyond the symbolism of the Union Jack flag being lowered in Brussels after 47 years, little will actually change until the end of 2020, by which time Johnson has promised to strike a broad free trade agreement with the EU, the world's biggest trading bloc.

'LET'S HOPE IT'S A SUCCESS'

The EU cautioned that leaving meant losing the benefits of membership, though the United States said Britons wanted to escape the "tyranny of Brussels". [nL8N2A02HF][nL8N2A02H9]

For proponents, Brexit is "independence day" - an escape from what they cast as a doomed German-dominated project that is failing its 500 million people.

"I voted to leave," said Mark Campbell, a 52-year-old writer. "Let's hope it's a success. I mean, also, 50 years is nothing."

Opponents believe Brexit is a folly that will weaken the West, shrivel what is left of Britain's global clout, undermine its economy and ultimately lead to a more inward-looking and less cosmopolitan set of islands in the northern Atlantic.

"It's a very sad day," said Roger Olsen, a 63-year-old engineer. "I think it is a disaster. An absolutely wrong thing. And I think time will prove that we have taken the wrong course."

Brexit was always about much more than Europe.

The referendum, which split voters 52% to 48%, showed up deep divisions and triggered soul-searching about everything from secession and immigration to empire and modern Britishness.

So severe was the Brexit meltdown that allies and investors were left astonished by a country that for decades had seemed a confident pillar of Western stability.

DISUNITED KINGDOM

At home, Brexit has tested the ties that bind the United Kingdom: England and Wales voted to leave the bloc but Scotland and Northern Ireland voted to stay.

Scotland's First Minister, Nicola Sturgeon, used the moment to demand a second independence referendum. A poll on Thursday suggesting a slim majority of Scots would now back a split because of Brexit. [nL8N29Z602]

So on "Brexit Day", some Britons will celebrate and some will weep -- but many will do neither. Many are simply happy that years of political wrangling about the divorce are over.

"To be honest, I think it's been going on for so long now, I just wanted to see it done with," said Lee Stokes, a 44-year-old project manager.

Brexiteers hope departure will herald democratic and economic reforms to reshape Britain and propel it ahead of its European rivals, which they say are chained to a doomed single currency.

Supporters of membership say Britain will atrophy and have little option but to cosy up to U.S. President Donald Trump. A Times newspaper cartoon had Johnson leaping out of the EU frying pan into the fire of Trump's orange hair.

"Britain’s place in the world will change," said outgoing opposition Labour Party leader Jeremy Corbyn. "The question is what direction we now take."

Across the EU, citizens bade farewell with sorrow or hope of a return, but also some support for Brexit - especially in Greece and Poland.

Johnson was chairing a cabinet meeting in Sunderland, the first city to declare a majority of votes for leaving the EU. Brexiteers will celebrate on Parliament Square.

A Union Jack in the building of the European Council in Brussels will be lowered at 7 p.m. (1800 GMT) on Friday, and put away with the flags of non-EU countries.

"We should have done it a long time ago," said Helen Brown, 50, in Dagenham, east London. "I'm glad Boris has finally pulled his finger out. It will be tough at first. But I think it will be good for the country in the long run."

(Writing by Guy Faulconbridge; Additional reporting by Kylie MacLellan and Andrew MacAskill in London; John Chalmers and Gabriela Baczynska in Brussels; Editing by Kevin Liffey)

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