Data from the 382 local voting areas reportedly showed the provisional size of the UK (plus Gibraltar) electorate at record of 46,499,537. The previous record UK in a similar franchise was for the UK Parliamentary General Election in 2015, with the 46,354,197 voters registered prior to the vote.

As with British parliamentary elections, Irish citizens living in Britain - an estimated 500,000 people - are allowed to vote, as are Commonwealth citizens residing in the UK, including nationals of European Union member states Cyprus and Malta. Other EU nationals, however, are not allowed to cast a ballot in the referendum.

Last-minute campaigning

UK Prime Minister David Cameron meanwhile said that the referendum was likely to be a close call. In the event of a "remain" vote, however, Cameron predicted an investment surge into Britain.

"It's very close; nobody knows what's going to happen," he told The Financial Times newspaper in an interview.

In a final push, Cameron also appealed to older voters especially to think about the impact leaving the European Union would have on the country and future generations. Opinion polls have shown a clear generational divide in opinion, with younger voters more in favor of staying in the EU and a higher number of older voters favoring a so-called Brexit.

ss/kl (AFP, Reuters UK Electoral Commission)