Financial markets and bookmakers put their money on Britain's voting to remain a part of the European Union on Thursday in a historic referendum that threatens to undermine the experiment in continental unity launched in the aftermath of World War II.

More than 46 million people were 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 and question 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, but the pound surged Thursday amid market optimism that uncertainty over the vote would end with a vote to stay.

The pound, which experts say would face a sharp drop in case of an EU exit, rose to its highest level this year, to around $1.4900. Britain's benchmark stock index was up 1.2 percent, adding to big gains so far this week. Other European stock markets were up almost 2 percent.

The bookies also clearly saw momentum to "remain." The betting market Betfair said the probability that the country will stay stands at 86 percent, with a British exit, or Brexit, given just a 14 percent chance. William Hill revised the chances of a "leave" vote down from 25 percent to just under 17 percent.

Also buoying the markets was an opinion poll conducted by the Ipsos MORI firm for the Evening Standard newspaper which suggested the "remain" camp winning by a narrow margin.

"It's perfectly possible that England will vote to leave but London as the capital city, and Scotland and Northern Ireland will all vote to remain and outweigh ultimately the view of the English as a whole," said Ben Page, the chief executive officer of the market research firm.

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. Those favoring "leave" also tend to be more committed.

"It's all about turnout and those soft `remainers' staying at home," U.K. Independence Party leader Nigel Farage said outside his home after collecting the newspapers.

"I do think we are in with a very strong chance, I do genuinely," he said.

It was raining heavily in some parts of the country, which could reduce turnout and there were anecdotal reports of voters being unable to get to the polls.

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.

Weather forecasters quipped that voters in the southeast would need umbrellas and possibly rubber boots in order to cast ballots. Prime Minister David Cameron ignored questions about the weather as he and his wife, Samantha, cast their votes in London.

"I think it's very important to vote in such a big decision," said voter Belinda Byrne in Teddington, southwest London.  "It's going to have a huge impact on the country."

The slaying of pro-Europe lawmaker Jo Cox in the final week before the vote may also prove a factor in the outcome.  While the motive is unclear, the rare killing of a politician cast a shadow over a divisive campaign unusually heated even by Britain's boisterous standards.

Her husband, Brendan Cox, told a crowd of some 9,000 people gathered in Trafalgar Square on the eve of the vote that his wife had died because of her beliefs,

"Jo's killing was political," he said. "It was an act of terror designed to advance an agenda of hatred toward others. What a beautiful irony it is that an act designed to advance hatred has instead generated such an outpouring of love."

Only hours before her death, the "leave" campaign had rolled out a poster showing hundreds of non-white migrants making their way across Europe, alongside the words "BREAKING POINT." Critics labeled the poster racist; Farage apologized that it was "used by those who wish us harm," but insisted he couldn't "apologize for the truth."

The reach of the EU into every aspect of life has made the issues at stake far more complex than in a general election -- and far more heated. Unlike in an election where the results can be reversed in the next term, the vote is final.

There is a small chance, though, that Parliament could choose not to implement the decision. But such a move would be politically difficult -- as any politician would find it tough and likely a career-ending move to go against the express will of the people.