Britons were voting on Thursday on whether to stay in the European Union in a referendum that has split the nation and is being nervously watched by financial markets and politicians across the world.
Opinion polls taken before the vote indicated the outcome is too close to forecast, although two polls published late on Wednesday suggested a swing towards "Remain".
Much will depend on turnout, with younger Britons seen as more supportive of the EU than their elders but less likely to vote.
An estimated 46,499,537 people are entitled to take part in the vote - a record number for a UK poll. Polling stations opened at 07:00 local time and will close at 22:00. Opinion polls have suggested that while big business is broadly in favour of staying in the EU, small firms have been evenly split in what looks like a photo-finish with one poll showing "Remain" at 45 % and "Leave" 44 %, with 11% undecided.
Prime Minister David Cameron on Wednesday made a last-minute plea to British voters not to give up the "best of both worlds" by voting to remain in the EU. Leading 'Brexit' backer Boris Johnson, a former London mayor who is widely touted as a future prime minister, insisted the "Leave" campaign was on the brink of victory.
Immigration to Britain, which has risen significantly in recent years, has been a key issue that has seriously divided the country. It is only the third nationwide referendum in UK history and comes after a four-month battle for votes between the "Leave" and "Remain" campaigns.
The referendum ballot paper asks the following question: "Should the United Kingdom remain a member of the European Union or leave the European Union?"
People hold signs in Trafalgar Square during a Yes to Europe rally for young people, ahead of Thursdays EU referendum, in central London, Britain June 21, 2016.Credit: Reuters
Whichever side gets more than half of all votes cast is considered to have won. The weather forecast for polling day is mixed as a rain-swept nation votes. After the referendum polls close, sealed ballot boxes will be collected and transported to the count venue for each of the 382 local counting areas, the BBC reported.
These represent all 380 local government areas in England, Scotland and Wales, plus one each for Northern Ireland and Gibraltar. Individual areas' results will then be declared throughout the night, along with results from 11 regional counts.
Depending on how close the poll is, the result may become clear before the final national result is officially declared by the Chief Counting Officer, who will be based at Manchester Town Hall. The Electoral Commission estimates a final result "around breakfast time" on Friday.
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