LONDON >> Voters in Britain are deciding Thursday whether the country should remain in the European Union — a historic vote that has exposed deep divisions over issues of sovereignty and national identity.

More than 46 million people are registered to vote in the referendum, which asks: “Should the United Kingdom remain a member of the European Union or leave the European Union?” The heated campaign saw the nation take stock of its place in the modern world, even as it questioned the direction it wanted to take in the future.

“This is, I’d say, the most important day in the past 20 years, at least for the U.K., and the economic consequences of a vote out are huge,” said investment banker Hasan Naqvi outside a London polling station.

“Leave” campaigners claim that only a British exit can restore power to Parliament and control immigration. The “remain” campaign led by Prime Minister David Cameron argues that Britain is safer and richer inside the 28-nation EU.

Financial markets have been volatile ahead of the vote, with opinion polls suggesting a tight race. The pound has surged over the week amid market optimism that uncertainty over the vote would end with a vote to stay. The pound briefly hit $1.48 in overnight trading, the highest level since the beginning of the year.

Turnout is considered critical in the vote, as polling suggested there were a number of undecided voters. A large turnout will favor the “remain” campaign as those who waver at the end tend to go for the status quo.

It was raining heavily in some parts of the country, which could reduce turnout. Downpours and flooding swamped parts of London and southeastern Britain. London’s Fire Brigade received hundreds of calls of weather-related incidents early Thursday, including some reports of flooding and lightning strikes.

Polls are open until 11 a.m. HST Thursday, with results due early Friday.