London: The UK could be 24 hours away from voting to leave the European Union, a move that would send shockwaves through the continent's politics and the global economy.

In a last minute intervention, the president of the European Commission warned that "out is out" and there would be no new treaty negotiation with Britain if the country voted to leave the EU.

A pro-EU banner being towed across London on Wednesday. Photo: Alastair Grant/AP

"The British policy makers and British voters have to know that there will not be any kind of renegotiation," Jean-Claude Juncker told reporters in Brussels.

After four months of an often bitter and rancorous campaign, touched last week by the Jo Cox tragedy, the Leave and Remain groups spent the final day before the poll in a blur of desperate last-minute vote-seeking.

Polls have the sides at a statistical dead heat, though most pollsters predict a narrow Remain vote if voters baulk at the last minute at changing the status quo: a pattern common to previous big referendums in Scotland and Quebec.

British Prime Minister David Cameron may have his job riding on the outcome of Thursday's vote. Photo: Getty Images

"A Remain victory is the most likely outcome, but it is far from guaranteed," YouGov's Joe Twyman told Buzzfeed.

ComRes' Andrew Hawkins said: "One week before the referendum it looked like Leave were going to nail it, but their momentum has been stopped in its tracks. The numbers now favour Remain."

However all pollsters agreed turnout would be the key: the more voters took to the polls on Thursday, the more likely a Remain result. Vice versa, if voters stayed home, the more motivated Leave campaign would have the edge.

The result could affect the people of Gibraltar more than many. The British outpost at the bottom of Spain may find its border with Europe shut if the UK votes to leave the EU. Photo: Sean Gallup/Getty Images

Immigration and the economy have been the two major themes of the campaign.

Remain argued that leaving the EU would trigger a recession, capital flight and job losses – a mantra their opponents dubbed "Project Fear".

Leave pushed for "taking back control" of Britain's laws and borders, emphasising the burden that EU immigrants placed on the UK – a theme London mayor Sadiq Khan called "Project Hate".

UKIP leader ad Leave campaigner Nigel Farage has been accused of using fear to rally Leave voters. Photo: Alastair Grant/AP

On Tuesday night it emerged the third-biggest individual donor to the Leave campaign was a former member of the racist British National Party.

On Wednesday, leading Leave campaigner Boris Johnson pushed his "take back control" message, urging voters to "believe in our country".

"It's time to have a totally new relationship with our friends and partners across the Channel," he said.

"It's time to speak up for democracy, and hundreds of millions of people around Europe agree with us. It's time to break away from the failing and dysfunctional EU system."

UKIP leader Nigel Farage said the Leave campaign had harnessed "real passion" in the country.

"Our supporters would crawl over broken glass to get down to the voting booth tomorrow," he said.

Prime Minister David Cameron said the UK would be "stronger, safer and better off" if it stayed in the EU, urging fellow activists for one more day to "hammer out that message".

Former prime minister John Major dubbed the Leave campaign the "gravediggers of our prosperity", warning that an out vote would diminish Britain's influence on the world.

And opposition Labour leader Jeremy Corbyn said Remain would protect millions of jobs that depended on exports and trade with Europe, and would help defend workers' rights which were guaranteed by the EU.

"To remain will help us to be able to reform the European Union and will be right for people in this country," he said.

Scotland's First Minister Nicola Sturgeon hinted at another referendum north of the border in the event of a Brexit vote, saying the Scottish government would be "talking directly to our European partners about how we protect our place in Europe and in the single market".

Brendan Cox, husband of Labour MP Jo Cox who was murdered last week in what police and the courts are treating as an act of political terrorism, appeared at a gathering in Trafalgar Square in memory of his wife.

"Our world collapsed on Thursday," he told the crowd. "I wish I wasn't here today – I'd rather be with Jo."

Wednesday would have been Mrs Cox's 42nd birthday and she would have spent it dashing around her electorate campaigning for Remain, he said.

"She feared the consequences of Europe dividing again, hated the idea of building walls between us and worried about the dynamics that could release," he said.

Polls close 10pm on Thursday in Britain – 7am Friday on the east coast of Australia – and results from 382 local centres will start to roll in about three hours later.

By the time Britons are eating their breakfast, they will know the result.

David Cameron is then expected to make a statement outside Number 10 Downing Street.

Many have predicted a Brexit vote would trigger a leadership spill in the Conservative party, and likely a new prime minister.

However Mr Cameron has said he would stay on and implement the decision of the British public.

The Brexit process would begin with the government officially notifying the EU of its wish to leave, triggering a two-year process under which the exit conditions are negotiated.

However the decision may have to be ratified by the British parliament – of which a big majority of MPs are pro-Remain.