British voters were deciding Thursday whether or not their country should leave the European Union, with polls before the vote putting the Remain side ahead in a referendum that could send shockwaves around the 28-country bloc and the world.

One of the last opinion polls before voting opened put the Remain vote on 55 per cent and Leave on 45 per cent. Populus surveyed 4,700 people online on Tuesday and Wednesday.

Two other polls published late Wednesday gave Remain a clear lead but found that up to 12 per cent of voters remained undecided.

Voting at 41,000 polling stations was taking place as torrential rain caused flooding in London and the south of the country, bringing travel headaches and soaking people as they went to cast their ballots.

Five of the 3,754 polling stations in London opened late, including two that were relocated because of flooding, local officials said.

The weather could have an effect on the so-called Brexit referendum, with results expected to be close. Support for Remain is relatively strong in London, according to polls.

The first indication of which side might have won the vote will come immediately after polling stations close their doors at 10 pm (2100 GMT) with the publication of an online survey of voting decisions by YouGov for Sky News.

Preliminary turnout data is scheduled to follow around 11:30 pm (2230 GMT).

The first local results are expected in the early hours of Friday and the final result around 7 am. About 46.5 million people were registered to vote.

Bookmakers have also given Remain a higher chance of victory than Leave.

"The bookies usually get it right," British Labour chief Jeremy Corbyn said as he arrived to cast his ballot in London.

But Nigel Farage, leader of the anti-EU UK Independence Party, told the Press Association he thought the Leave campaign still had "a very strong chance."

"But it's all about turnout and those soft Remainers staying at home," Farage said outside his home in Kent.

Although political rivals, opposition chief Corbyn and conservative Prime Minister David Cameron had both campaigned separately for Britain to stay in the EU. Many other senior Labour figures joined Cameron at Remain campaign events.

Much of the debate focussed on the economy and EU migration, particularly in the last few weeks before the referendum.

Brexit backers, spearheaded by Conservative rebel Boris Johnson and, in a separate campaign, by UK Independence Party leader Nigel Farage, urged voters to "take back control" of Britain's sovereignty and control EU migration by leaving the EU.

The Remain camp argued that an exit from the EU's common market would hurt the British economy and would cost jobs, a view that is shared by other EU countries, as well as by leading economists around the world.

In addition, financial markets could be thrown into turmoil if Britons vote the EU's second largest economy after Germany out of the bloc.

It would be the first time that a country decided to leave the EU.

However, market sentiment was still positive Thursday as the British pound reached its highest level of 1.48 dollars, and European stock indices were up.

"The markets are clearly anticipating a victory for the opponents of Brexit," said Jochen Stanzl, a Frankfurt-based analyst at CMC Markets.

In an attempt to calm markets in the case of a Brexit, dpa has learned that the finance ministers of the G7 group of leading industrialized nations are preparing a joint statement.

The ministers are working in coordination with their countries' central banks and will release the statement from Tokyo shortly after the referendum's result is known.

Cameron, under pressure from those in his own Conservative party and a rising euroscepticism among British voters, had promised in 2013 to hold a referendum on EU membership.

He called the referendum in February after negotiating a package of reforms with the bloc that he said gave Britain a "special status."

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