Businessman Arron Banks has described the referendum on the European Union as "an establishment stitch-up" and threatened to launch a judicial review if the government allow an extension of the voter registration deadline.
The registration website for voting in the EU referendum crashed for an hour-and-a-half before before Tuesday evening's deadline. The crash was put down to a last-minute rush of people registering to vote.
The government is expected to push through emergency legislation today to extend the deadline and ensure tens of thousands of voters do not miss out on the opportunity to have their say in the 23 June ballot.
Banks, a former Conservative supporter who has donated £1m to UKIP and is funding Leave.EU, argues the extension the government is pushing for should only be allowed for the 'missing' 90 minutes.
"It's more to do with the fact the rules are in place to check the validity of who's applying," he told Julia Hartley-Brewer.
"If you've got a mass surge like this, you can't possibly do [those checks] before the election. But the deadline was put in place and you can't change it mid-stream.
"From our point of view, the Electoral Commission is very pro-European to start with.
"They've spent huge amounts of money on youth registration as opposed to elderly people. They also allowed a £9m-pound leaflet and have spent four times as much on the referendum than the general election.
"It's been an establishment stitch-up at every level."
If the legislation to extend the registration deadline is passed, Banks has said he will pursue a judicial review to try to change it.
"It's a tricky area because this has never happened before," he said.
"No one has ever tried to change the rules of the game in the middle of the game."