Brexit polls close as Europe holds its breath

Millions of Britons have now voted on whether to stay in the EU, with the final opinion poll pointing to a slender victory for the "Remain" campaign.

The polls closed at 7am (AEST), after a close-fought and often vitriolic campaign focused on immigration, the economy and Britain's very identity.

A YouGov survey found 52 percent were backing Britain's membership of the European Union, and 48 percent wanted to leave.

"The results are close and it's too early to call it definitively," said Joe Twyman, YouGov's head of political and social research.

"But these results, along with the recent trends and historical precedent, suggest a Remain victory is the more likely outcome."

A record 46.5 million people registered to vote in the once-in-a-generation referendum being nervously watched across Europe and the world.

European leaders fear a so-called Brexit would trigger the biggest crisis in the 60-year history of post-war integration on the continent.

London's benchmark FTSE 100 share index closed up 1.2 percent at 6,338.1 points, while key eurozone indices in Frankfurt and Paris forged even higher, at around two percent up.

An Ipsos MORI opinion poll for the London Evening Standard newspaper put "Remain" on 52 percent and "Leave" on 48 percent.

Australian fallout

Foreign Minister Julie Bishop has declared Australia well placed to withstand any economic fallout from a a potential British departure from the European Union.

"If Britain decides to leave it will take quite some time for the conditions of the exit to be negotiated I would imagine," Ms Bishop told the TODAY Show, adding that would give Australia plenty of time to consider a response.

The Australian dollar has soared to a two-month high with odds that Britain will vote to stay in the European Union increasing.

Turnout crucial

Jenny Watson, who chairs the Electoral Commission referendum overseers and will announce the official result, said recounts would not take place "simply because the vote was close".

"If it's a dead heat then it's a dead heat. There is no casting vote," she told the BBC.

While early indications of results could come from around 12pm Friday (AEST), the final declaration is not likely until around 6pm (AEST).

Leave figurehead Boris Johnson, the former London mayor, said the race was "very close", as he returned to the British capital from Edinburgh, but added: "From what I have heard... turnout is good in areas where we need it to be."

In London and southeast England, many voters braved torrential rains and thunderstorms to have their say.

Several polling stations had to be relocated due to flooding and one was being run on a generator due to a power outage.

The referendum asks: "Should the United Kingdom remain a member of the European Union or leave the European Union?"

Commentators suggest Brexit could trigger a constitutional crisis in Britain, prompting another Scottish independence referendum.

There are also fears it could prompt other EU countries, disillusioned with how Brussels has handed the eurozone and migrant crises, to try to break away.