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 per cent, 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 per cent, according to Betfair odds, and the FTSE 100 share index was up 1.2 per cent 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 $US2.9 trillion ($A3.89 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 (1100 AEST).
Brexit referendum Source: AFP
Source: AFP
另外德国总理默克尔日前强调，她希望英国选民投下留欧票。她与波兰总理斯兹德罗（Beata Szydlo）会谈后告诉记者：“当然我希望英国留在欧盟，但这要由英国人民决定。” 斯兹德罗说，她也“强烈希望”英国人民选择留在欧盟。
公投将于当地时间23日上午7时开放到晚间10时，投票结果预计24日清晨就会出炉。官方不会公布出口民调，但一些避险基金和银行据说已委托民间民调公司进行出口民调，这样他们就可以得知投票状况，在投票还在进行时作为交易依据。
包括直布罗陀在内，计票所共382处，各计票所一有结果，即可自行宣布。率先宣布结果的预料会是北英格兰的巽德兰（Sunderland）与伦敦万兹渥斯区（Wandsworth），24日0时30分就会有结果。第一波结果出炉要到24日凌晨2时，另一波则会等到数小时后。投票率会较早公布，可初步窥知投票结果。若投票率高，对「留欧」阵营有利。
