British Prime Minister David Cameron and his eurosceptic opponents are making their final pitches on the eve of the Brexit referendum.
The vote on Thursday, which echoes the rise of populism elsewhere in Europe and the United States, will shape the future of Europe.
A victory for "out" could unleash turmoil on financial markets.
"It's very close; nobody knows what's going to happen," Cameron told Wednesday's Financial Times.
The Leave camp was on 45 percent, just one point ahead of Remain, with nine per cent undecided, in the last poll published by Opinium, which called it a "statistical tie".
European Commission chief Jean-Claude Juncker warned there would be no further renegotiation whatever the result, after EU leaders reached a deal on a new settlement for Britain in February.
Much of the heated debate has boiled down to two issues: the economy and immigration.
In what has become a bitter fight, both camps have been accused of using unfounded assertions and scare tactics.
The City of London financial centre, the International Monetary Fund and the majority of British business leaders back Cameron and his Remain camp's stance that to leave the EU would plunge Britain into recession, costing jobs and raising prices.
Brexit supporters say Britain would regain control of immigration.
Former London mayor Boris Johnson, the main leader of the Leave campaign and favourite with bookmakers to replace Cameron in the event of Brexit, is flying around Britain in a helicopter to spread the Brexit message, making an unashamed play to British patriotism by declaring it "independence day".
Opinion polls have depicted a deeply divided nation, with big differences between older and younger voters, and between pro-EU London and Scotland and eurosceptic Middle England.
Election experts say turnout will be crucial because of a gulf between generations.
Opposition Labour Party supporters are seen as being more pro-EU than Conservatives, but less passionate about the issue.
The implied probability of a Remain vote was at 75 percent, according to Betfair odds, and the FTSE 100 share index was up 1.2 percent as investors grew more optimistic Britain would back staying in the EU.
World leaders including US President Barack Obama, Chinese President Xi Jinping and NATO and Commonwealth allies have urged Britain to remain in the EU.
A Brexit vote could unleash turmoil on markets, lead to a political crisis in Britain and fragment the post-Cold War European order.
EU leaders unanimously urge Britain to stay but are debating whether the right Brexit response would be closer integration or a rethink of the way Europe is governed.
The EU would have to weather the exit of its No.2 economy representing $US2.9 trillion of its gross domestic product, the only European financial capital to rival New York and one of its only two nuclear powers, while Britain's economy could stall.
Scotland's First Minister says Brexit could also trigger another independence referendum if Scots backed staying in the EU.
The official result is due Friday morning but partial results will be announced from about 0100 GMT.