LONDON - EU leaders warned Britain Wednesday that leaving the bloc would be final as two polls put the Brexit vote just ahead of "Remain" on the eve of a knife-edge referendum that has put the continent on alert.

The Opinium poll put the "Leave" camp at 45 percent and "Remain" at 44 percent, while TNS gave them a lead of 43 percent to 41 percent for staying, although both results were within the margin of error.

"Our latest poll suggest that Leave is in a stronger position than Remain," Luke Taylor, head of social and political attitudes at TNS, said in a statement.

But he cautioned that "a late swing to the status quo" was possible in the final hours before voting.

A Brexit vote would mean Britain would be the first country to leave the European Union in the bloc's 60-year history, leaving 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.

Merkel and Hollande will meet in Berlin next week for talks which the French president said would work "towards relaunching the European project", already struggling with an unprecedented migrant crisis.

Prime Minister David Cameron, who faces calls to resign if he loses, spent the final day of campaigning travelling around Britain on his battle bus and doing interviews.

"If I had to sum up this whole campaign in a word, it would be that word 'together'," the Conservative leader told BBC radio.

Out on the campaign trail, he added: "If we want a bigger economy and more jobs, we are better if we do it together".

Bosses from nearly 1,300 of Britain's leading businesses signed a letter in The Times saying the country was stronger in the EU.

James Bond star Daniel Craig and Irish rock band U2 became the latest celebrities to back "Remain".

Despite the polls showing the race is neck and neck, bookmaker Betfair said their latest odds implied a 76-percent chance of "Remain" winning.

On the eve of the vote, planes with banners from the rival campaigns criss-crossed the skies above central London trying to woo undecided voters.

Cameron's main rival in the "Leave" campaign and possible successor, Boris Johnson, said Britain stood on the brink of "independence day" from Europe.

"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," Johnson said in eastern England.

Nigel Farage, leader of the anti-EU UK Independence Party, said: "I genuinely believe we are going to win this."

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.

"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.

The "Leave" campaign briefly took a slight lead in many opinion polls until last week, sending sterling plummeting.

This fell away after campaigning was paused for two days following Thursday's killing of pro-EU lawmaker Jo Cox of the main opposition Labour party.

Wednesday would have been her 42nd birthday and a series of commemorative events were being held in Britain and around the world.

Thousands of people gathered in London's Trafalgar Square to pay tribute to Cox at an event addressed by Pakistani Nobel Peace Prize winner Malala Yousafzai.

A floral tribute to Cox was also towed along the River Thames to a mooring outside the Houses of Parliament.

Cox's widower Brendan has said his wife, who was particularly noted for her work on refugee rights, was killed because of her political views.