A record number of voters have registered for Britain's EU membership referendum on Thursday with 46.5 million people signed up to cast their ballots, the body overseeing the vote said.

"The provisional size of the UK and Gibraltar electorate indicates a UK record of 46,499,537," the Electoral Commission said in a statement on Tuesday.

The previous record was in last year's general election when the number was 46,354,197.

As with British parliamentary elections, Irish citizens living in Britain -- a population estimated at around 500,000 people -- are allowed to vote.

Commonwealth citizens, including nationals of European Union member states Cyprus and Malta, can also vote.

The commission said 24,117 people had registered in Gibraltar.

The deadline for registering was June 7 but this was extended by two days after a rush of last-minute sign-ups caused the official registration website to crash.

Around 132,000 of the 525,000 people who did successfully register on June 7 were aged under 25, compared to around 13,000 from the 65 to 74 age group.

Opinion polls have shown a clear generational divide, with younger voters more in favour of staying in the EU while older voters tend to favour a Brexit.

Younger people are also traditionally seen as less likely to vote, meaning that high registration numbers could favour the "Remain" side.