(CNN)Voters headed to the polls Thursday in ahistoric referendum on the United Kingdom's future membership in the European Union.

Citizens clutched umbrellas as they lined up at the polls despite a morning drizzle in London. Weather across the rest of the region was mixed -- sunshine was forecast in parts of Scotland while heavy showers were set to move across Northern Ireland.

While some schools were closed to serve as polling stations, it was class as usual for others.

A record number of people -- almost 46.5 million -- are registered to take part in the once-in-a generation vote.

The registered voters include Britons from England, Wales, Scotland, Northern Ireland and Gibraltar -- a British territory off the southern coast of Spain.

The question they've been pondering for months: Should the United Kingdom remain a member of the European Union or leave the European Union?

British citizens over age 18, along with Irish and Commonwealth citizens living in the UK, are expected to deliver their verdict at the ballot box Thursday. British citizens living abroad have already cast their votes by mail.

EU referendum: What does it mean to you?

Polling stations across the UK opened at 7 a.m. local time (2 a.m. ET) and will close at 10 p.m. (5 p.m. ET), with the first results expected about midnight (7 p.m. ET). Strict rules mean that broadcasters, such as CNN, are limited in what they can report while polls are open.

The final, nationwide result is expected to be announced Friday morning.

The UK has been a member of the European Union (and its precursors) since 1973.

European citizens living in the UK are not eligible to vote unless they're from Malta or Cyprus -- both members of the Commonwealth. Citizens of the Republic of Ireland are eligible to vote if they live in the UK. Members of Britain's House of Lords are also eligible to vote -- even if they cannot cast ballots in general elections.

Once the results are in, they will be announced by region rather than by constituency. The 12 regions comprise Scotland, Northern Ireland, Wales, London and eight English regions.