Voters headed to the polls Thursday in a historic referendum on whether the United Kingdom should leave the European Union.

Citizens clutched umbrellas as they lined up at the polls despite heavy rain and overnight thunderstorms causing flooding in parts of London and southeastern England.

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.

CNN crews reported brisk voting, and members of the public posted photographs of busy polling stations across the country.

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?

As soon as polling stations opened at 7 a.m. local time (2 a.m. ET), British citizens over age 18, along with Irish and Commonwealth citizens living in the UK began delivering their verdict at the ballot box.

They included UK Prime Minister David Cameron, who voted at a hall in London alongside his wife, Samantha Cameron.

British citizens living abroad have already cast their votes by mail.

Some schools across the UK are closed for the day, to serve as polling stations, but it is class as usual for others. (CNN)

•British PM David Cameron and his wife Samantha leave a polling station in London.