A man wearing a European themed cycling jersey leaves after voting at a polling station for the Referendum on the European Union in north London, Britain, June 23, 2016. [Photo/Agencies]

Results are due to be announced throughout the night.

The vote came on a day when London and parts of southeast England were hit by torrential rain, causing floods and widespread transport chaos.

Five London polling stations opened late as staff struggled to get there, and two closed briefly because of flooding but were re-opened in back-up locations. Local media reported some voters had to wade through water to reach a polling station.

"In London/southeast and want to vote in the #EURef? Make sure you plan now to get back to your local polling station by 10pm!" the Electoral Commission said during the evening on Twitter as commuters struggled with train cancellations.

Among those affected was Johnson, who cast his vote with just 25 minutes to spare after returning to the capital from his daughter's graduation in Scotland.

"Let's see, let's see. It's in the hands of the people now," he said when asked how he felt about the vote.