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 (0200 EDT) and will close at 2100, with results expected to be announced by the 382 individual local counting areas between around 0100 and 0300 on Friday.
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.
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 murder 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.