Prime Minister David Cameron invoked Britain's wartime spirit in a last-ditch bid to win votes on Wednesday on the eve of a knife-edge referendum on European Union membership that has put the continent on alert.
"Winston Churchill didn't give up on European democracy... and we shouldn't walk away," David Cameron told a crowd in Birmingham, his final rally in a campaign that has been described as one of Britain's most bitter ever.
EU leaders warned that leaving the 28-member bloc would be final, with just hours to go until a vote in which record numbers of Britons have registered to cast a ballot.
"If you jump out of the airplane, you cannot clamber back through the cockpit hatch," Cameron warned, his sleeves rolled up and pointing for emphasis.
"Put your children's future first."
The Thursday editions of British newspapers captured the drama of voting day. "Independence day" was the headline of the pro-Brexit Sun, while the Times called it a "Day of reckoning".
Two polls showed "Leave" had the slimmest of leads, but a third showed the "Remain" ahead and betting markets indicated a pro-EU vote was more likely.
"With a race as close as this, the turnout level... will be critical," said Luke Taylor of pollster TNS.
A "Leave" victory would make Britain the first country to exit the European Union in the bloc's 60-year history, putting it in uncharted waters at an already troubled time.
"Out is out," European Commission chief Jean-Claude Juncker said in Brussels, dismissing any talk of a post-vote renegotiation just hours before polls open.
French President Francois Hollande warned an exit would be "irreversible" while German Chancellor Angela Merkel said she wanted Britain to stay but that the decision was down to the British people.
The German and French leaders will meet in Berlin next week for talks Hollande said would work "towards relaunching the European project", already struggling with an unprecedented migrant crisis.
- Domino effect -
A final television debate underlined the muddied picture as a mix of politicians, television personalities and ordinary people squabbled over the pros and cons.
"I came here confused and I'm more confused," admitted glamour model Katie Price after the politicians had laid out their arguments.
Earlier, planes with banners from the rival campaigns flew over London to woo the undecided.
"I do think that we are on the verge, possibly, of an extraordinary event in the history of our country and indeed in the whole of Europe," said Boris Johnson, Cameron's main rival in the "Leave" campaign and possible successor.
US Republican White House hopeful Donald Trump, who arrives in Britain Thursday, also spoke out on Brexit again, saying he thought the country should "go it alone".
A British withdrawal would trigger a lengthy exit negotiation, leading to the loss of unfettered access to its partners in the 28-nation market and forcing the country to strike its own trade accords across the world.
In Europe, the referendum has raised concerns of a domino effect of exit votes that would imperil the integrity of the bloc, already buffeted by the eurozone and migration crises.
Though many voters fret over the financial consequences of a Brexit, others relish the prospect of taking back power from Brussels and reining in high levels of immigration.
- 'Act of terror' -
"I think we need to make our contribution to Europe and to the global economy. And the best way we can do that is by being in it," Chet Patel, a 44-year-old telecoms worker told AFP.
Pat Hand, a 50-year-old construction worker, said he would be voting to leave the EU. "The country is in an absolute mess," he added.
Momentum for the "Leave" campaign, however, appeared to be upended with last week's killing of pro-EU lawmaker Jo Cox of the main opposition Labour party, which prompted concerns the campaign had been divisive.
"Jo's killing was political. It was an act of terror," Cox's husband Brendan told thousands of mourners who gathered in London's Trafalgar Square to mark what would have been her 42nd birthday.
A floral tribute to Cox was also towed along the River Thames to a mooring outside the Houses of Parliament.
With everything to play for, a string of prominent figures from James Bond star Daniel Craig to rockers U2 rolled out last-minute endorsements.