British voters are going to the polls Thursday to vote on whether their country should remain a member of the European Union. As months of fierce debate pause on referendum day and Britons hold their breath for what has been described as a "once-in-a-lifetime" decision, AP explains the purpose and mechanics of the vote.

WHY WAS THE VOTE CALLED?

British Prime Minister David Cameron courted conservative and anti-EU voters during the last election by promising to hold a referendum on the UK's membership of the 28-nation bloc by the end of 2017. Those campaigning to leave say the EU has evolved into an undemocratic and oppressive entity far removed from its original purpose as a trading bloc that Britain originally joined in 1973. They claim that only a British exit — or Brexit — can restore sovereignty and effectively limit immigration. Those campaigning to remain argue that the EU ensures peace and prosperity for more than 500 million people from Portugal to Finland and the benefits far outweigh the costs.

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WHO CAN VOTE?

British and Irish citizens 18 and over who are resident in the United Kingdom, as well as U.K. residents from Commonwealth countries who have the right to live in the country can vote in the referendum. U.K. nationals who live outside the country but were registered to vote in Parliamentary elections in the past 15 years, and Irish citizens overseas who were born or registered to vote in Northern Ireland in the same period, can also vote. In addition, some citizens of Gibraltar — a British enclave on the south coast of Spain — and members of the House of Lords, who cannot normally vote in general elections, have been given permission to participate in the referendum. The Electoral Commission says a record number of 46,499,537 voters were registered for the referendum by Tuesday.

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WHAT'S ON THE BALLOT PAPER?

Voters are asked to answer one question: "Should the United Kingdom remain a member of the European Union or leave the European Union?" The options are: "Remain a member of the European Union" and "Leave the European Union."

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WHAT HAPPENS ON THE DAY OF THE REFERENDUM?

Polling stations open at 7 a.m. (0600 GMT) and close at 10 p.m. (2100 GMT). Many votes will have been cast in advance by postal ballot. Election officials in 382 areas will begin counting the votes immediately after polls close.

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IS THERE CAMPAIGNING ON VOTING DAY?

The law doesn't prohibit campaigning on voting day, but by convention political parties refrain from doing so. Publishing exit polls prior to the end of voting at 10 p.m. (2100 GMT) is, however, a criminal offense.

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WHEN AND HOW WILL THE RESULT BE ANNOUNCED?

Regional counting offices will send their results to Manchester, where the chair of the UK Electoral Commission is expected to announce the official outcome at about 7 a.m. (0600 GMT) Friday. However, the result may well be known as early as 4 a.m. (0300 GMT) as media tally the local results.

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WOULD THERE BE A RECOUNT IF THE VOTE IS CLOSE?

The rules don't allow for a national recount, but courts can order recounts at the local level. The overall outcome can be challenged by judicial review filed within six week.

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IS THE REFERENDUM BINDING?

No. Parliament isn't legally required to abide by the vote, but there would be strong political pressure to do so, especially if the result of the referendum is clear-cut.