LONDON, June 23 (Reuters) - Britons voted on whether to quit the European Union in a bitterly-contested referendum on Thursday that polarized the nation and could change the face of Europe.
Financial markets, on edge for weeks over the uncertain outcome, rose on the strength of late polls that showed a swing towards staying in, but the bulk of recent polls have suggested the outcome was too close to call.
If Britain becomes the first state to exit the EU, the so-called Brexit would be the biggest blow to the 28-nation bloc since its foundation. The EU would be stripped of its second-biggest economy and one of its two main military powers, and could face calls for similar votes by anti-EU politicians in other countries.
If it votes to stay, Britain has been promised a special status exempting it from further political integration but European leaders will still have to address a sharp rise in euroscepticism across the continent.
A Brexit vote would also deal a potentially fatal blow to the career of Prime Minister David 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.
After four months of campaigning, polling stations opened at 0600 GMT and were due to close at 2100, with results expected to be announced by the 382 individual local counting areas between around 0100 and 0300 on Friday.
The Leave campaign says Britain would recover full sovereignty and the economy would benefit from a Brexit. It focused its campaign on warnings that Britain would be unable to control immigration levels as long as it was an EU member.
Remain says a Brexit would cause financial chaos, impoverish the nation and diminish its influence on world affairs, emphasizing the economic benefits of membership and the risks posed by leaving.
The campaign was suspended for three days after the killing of pro-EU member of parliament Jo Cox a week ago, which prompted soul-searching about the vicious tone of the debate.
An Ipsos MORI poll for the Evening Standard newspaper found support for Remain on 52 percent and Leave on 48 percent. A Populus poll put Remain 10 points ahead on 55 percent. Both were conducted on Tuesday and Wednesday.
Cameron called the vote under pressure from the rebellious anti-EU wing of his Conservative Party and the surging UK Independence Party (UKIP), hoping to end decades of debate overBritain's ties with Europe.
Unless Remain wins by a wide margin, Cameron could struggle to repair the rifts in his party and hold on to his job. He has said he would stay in office but in the event of a vote to leave he is likely to face calls to resign.
The Evening Standard newspaper, citing party sources including ministers, reported that some senior Conservatives from both sides of the divide planned to swiftly back Cameron to stay in office, regardless of the outcome.
"The markets are jittery already. Nobody wants the added uncertainty of a resignation," the newspaper quoted an unnamed minister as saying.
Cameron voted early and said on Twitter: "Today's the day you can vote for a stronger future - with Britain leading, not leave, Europe. #VoteRemain"
His main rival, fellow Conservative former London mayor Boris Johnson, who is the favorite with bookmakers to succeed Cameron, tweeted: "Now is the time to believe in this country and #VoteLeave. Let's make today our Independence Day."
If Britons choose to leave, Scottish leader Nicola Sturgeon has suggested Scotland, where sentiment towards the EU is much more positive, may hold another a referendum on leaving the United Kingdom. Scots voted against independence in a plebiscite in 2014.
Traders, investors and companies were braced for volatility on financial markets whatever the outcome of a vote that both reflected, and has fueled, an anti-establishment mood also seen in the United States and elsewhere in Europe.
After the Ipsos MORI poll was released, sterling broke above $1.49 for the first time since 2015, although it later fell back and was at $1.48 at 1500 GMT. It had fallen almost to $1.40 last week when polls showed a surge towards Leave.
The likelihood of a Remain vote implied by Betfair betting odds stood at 86 percent, the firm said.
If Britain votes to leave, finance leaders from the Group of Seven leading economies will issue a statement stressing their readiness to take all necessary steps to calm markets, government officials with direct knowledge of the matter said.
Britain's AAA credit rating could swiftly be downgraded by Standard & Poor's if the Brexit camp prevails, S&P chief sovereign ratings officer Moritz Kraemer told German daily newspaper Bild.
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Britain is deeply divided on EU membership, with older voters favoring exit and younger voters wanting to stay. London and Scotland favor staying in, while much of middle England wants out.
The killing of Cox, a 41-year-old mother of two young children, stabbed and shot on a street in her electoral district in northern England, cast a pall over the final days of campaigning.
The man charged with her murder told a London court on Saturday when asked his name: "Death to traitors, freedom for Britain." He is due to go on trial in November.
Whatever the outcome of the vote, the focus on immigration to Britain, which has increased dramatically in recent years, could worsen frictions in a country where the gap between rich and poor has also been widening.
The nation's divisions were reflected in newspaper front pages. "Independence Day" was the front page headline of the Sun tabloid, Britain's biggest-selling newspaper, while the Daily Mirror warned "Don't take a leap into the dark."
The issue also dominated news bulletins far beyond Britain. In China, the Global Times, published by the ruling Communist Party's official People's Daily, warned Britain would lose its influence globally if voters backed Brexit.
Foreign leaders, from U.S. President Barack Obama to Chinese leader Xi Jinping, have called onBritain to remain in the EU, a message supported by global financial organizations, many company bosses and central bankers.
U.S. Republican presidential candidate Donald Trump has voiced support for a Brexit.
"I don't think anybody should listen to me because I haven't really focused on it very much, but my inclination would be to get out," he told Fox Business on Wednesday. "You know, just go it alone. It's a mess," he said of the EU.
International banks have warned that the value of the pound could fall dramatically if Britain votes to leave and traders expect markets to be more volatile than at any time since the 2008-09 financial crisis.
The "Out" campaign says a fall in the value of the pound would boost exports and has found support among some financial specialists and small businesses. It has urged voters to ignore what it calls the "establishment" which it says has the most to lose from Brexit.