Britons were voting on Thursday on whether to stay in the European Union in a referendum that could change the face of Europe and is being nervously watched by financial markets and politicians across the world. A British exit, or Brexit, would deprive the 28-member EU of its second-biggest economy and one of its two main military powers, sending political shockwaves across the continent. After four months of bitter campaigning, polling stations opened at 0600 GMT and will close at 2100 (5 p.m. ET), with results expected to be announced by the 382 individual local counting areas between around 0100 and 0300 on Friday (9 p.m. to 11 p.m. ET Thursday). Prime Minister David Cameron, who is for "Remain," called the vote under pressure from the anti-EU wing of his Conservative Party and the surging UK Independence Party (UKIP), hoping to end decades of debate over Britain's ties with Europe. Even with a vote to stay, Cameron could struggle to repair the rifts in his party and hold on to his job.

Brexit supporters with signs in London, June 22, 2016. Michael Newberg | CNBC

The "Leave" campaign says Britain's economy would benefit from a Brexit. "Remain" says it would cause financial chaos and impoverish the nation for years or even decades to come. Two polls conducted on Tuesday and Wednesday found "Remain" was in the lead, although the overall picture from the last few days of polling was of a vote that was too close to forecast. An Ipsos MORI poll for the Evening Standard newspaper found support for "Remain" on 52 percent and "Leave" on 48 percent. A Populus poll found "Remain" 10 points ahead on 55 percent. Cameron voted early, and said on Twitter: "Vote Remain - so that our children and grandchildren have a brighter future." His main rival, 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." It is only the third referendum in British history. The first, also about membership of what was then called the European Economic Community, was in 1975. The campaign, which has exposed bitter divisions in the ruling Conservative Party, was dominated by immigration and the economy and shaken by the slaying of pro-EU Labour lawmaker Jo Cox last week. If Britons choose to leave, Scottish leader Nicola Sturgeon has suggested Scotland, where sentiment towards the EU is much more positive, may call a referendum on leaving the United Kingdom. STERLING UP 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. Sterling gained 1.5 percent against the dollar on Thursday, breaking above $1.49 for the first time since December 2015 after the Ipsos MORI poll was released. The likelihood of a "Remain" vote implied by Betfair betting odds stood at 86 percent, the firm said shortly after the poll.

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 be swiftly downgraded by Standard and Poor's if the Brexit camp prevails, S&P chief sovereign ratings officer Moritz Kraemer told German daily newspaper Bild. On Wednesday, campaigners from both sides tried to win over the estimated 10 percent of the 46.5 million electorate who polls suggest had still not decided which way to vote. The "In" campaign took aim at their rivals by saying a Brexit would hurt the economy, security and the country's status. The "Out" campaign said high levels of immigration could not be controlled inside the EU and it was time to bring powers back from Brussels to London. "STAY WITH US" Britain is deeply divided on EU membership, with big differences between older and younger voters, and between pro-EU London and Scotland and eurosceptic middle England. The killing of Cox, a 41-year-old mother of two young children, in her electoral district in northern England prompted a pause in the campaign and soul-searching about its tone. Cox's husband said she had been concerned about the coarsening of political dialog. The man charged with her killing 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.