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, 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.