A pro-Brexit businessman has threatened to legally challenge the result of the EU referendum after voter registration was extended.
The website for registering voters crashed shortly before Tuesday's original midnight deadline as thousands made a last-minute effort to sign up.
The government is introducing emergency legislation to extend the deadline and ensure more people can vote in the June 23 poll.
Leave campaigners argue the move is an attempt "to rig the referendum" by getting more young people signed up who are more inclined to back Remain.
Arron Banks, who backs the Leave.EU campaign, is considering launching a judicial review.
This isn't some democratic initiative, it's a desperate attempt by the Establishment to register as many likely Remain voters as possible before polling day," he said in a statement.
"We believe It is unconstitutional at best and have been advised that with legitimate cause we could challenge this extension.
"We are therefore considering all available legal options with our legal team, with a view to potentially launching a judicial review now and after the outcome of the referendum on 23rd June."
Vote Leave, the official Leave campaign in which Banks plays no part, has avoided making public criticisms of the move.
Michael Gove, a leading Brexiter, said while it was a step into "complex legal waters", he believed "everyone possible" should be given the chance vote.
Downing Street said there was no way the government could know the voting intentions of those applying to register.