Supporters of the Stronger In Campaign react after heading the result from Orkney in the EU referendum at the Royal Festival Hall, in London, Britain June 24, 2016. [Photo/Agencies]

A vote to leave would send decades of European integration into reverse, marking the first time an independent nation has broken away. It would threaten to open further cracks in a grouping already reeling from successive crises over Greek debt and a mass influx of refugees from Syria and elsewhere.

A vote to stay would leave the EU intact, with its most free-market proponent still a member. However, what began as a domestic political gambit by Cameron has polarised the country and exposed wider challenges facing Europe: public angst over immigration and the falling living standards of many in the world's richest region.

Marred by the murder of a pro-EU UK politician, Jo Cox, who was shot and stabbed in the street a week ago, the campaign and its divisive rhetoric highlighted the populist wave also seeping into the U.S. election race.

A defiant Farage said he was not conceding defeat, even though he feared the result would go against him.

"I hope I am wrong, I hope I am made a fool of, believing that to be the case. Either way, whether I am right or wrong, if we do stay part of this union it is doomed, it is finished anyway. If we fail tonight it will not be us that knocks out the first brick from the wall, it will be someone else," he said.

If it votes to stay, Britain has been promised a special status exempting it from any further political integration, but European leaders will still have to address a sharp rise in euroscepticism across the continent.

A Brexit vote, however, would deal a potentially fatal blow to the career of Cameron, who called the referendum and campaigned for the country to stay in, against a Leave camp led by rivals from within his own Conservative Party.

"Thank you everyone who voted to keep Britain stronger, safer and better off in Europe - and to the thousands of Remain campaigners around the UK," Cameron said on Facebook.

In a letter, 84 eurosceptic Conservative lawmakers called on Cameron to remain prime minister regardless of the result. It marked the first attempt to heal the deep rifts that have opened up in the ruling party since the start of the campaign.

The signatories included prominent Leave campaigners Boris Johnson, the former mayor of London, and Michael Gove, a cabinet minister and personal friend of Cameron.

But despite the statement of loyalty, Cameron would face huge pressure from the country at large to step down as prime minister if Britons have defied him and voted to leave.