Leave.EU, Arron Banks' project, has said it is considering legal action against the government after it was announced that the registration deadline for voters would be extended until midnight tonight.
While most called on the government to extend registration so that everyone had the chance to vote, Leave.EU thinks something rather underhand is going on.
"For the government to alter election law during an election period is absolutely unprecedented and unconstitutional," said Banks, co-chairman of the Leave.EU campaign. "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."
Banks said that it follows the Electoral Commission issuing thousands of ballots to EU nationals, while turning a blind eye to councils issuing postal voting guidance suggesting voters put their cross in the box for Remain and doing nothing about the Prime Minister's £9m pro-EU leaflet.
"Taken together, we believe that the above constitutes a clear attempt to rig the referendum or, at a bare minimum, to load the dice," Banks continued.
"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."
However, pro-Leave campaigner Michael Gove, of Vote Leave, said that "everyone possible" should be given the chance to vote, even though it was a step into "complex legal waters".
Though it's not the first time Leave campaigners have threatened legal action. Vote Leave made a veiled threat to ITV that there will be "consequences" to what the group claimed was a Remain bias after Ukip leader Nigel Farage was selected to debate David Cameron.
And Leave.EU had previously threatened to take legal action against the Electoral Commission after it was announced Vote Leave would be the officially designated campaign group for pro-Brexiteers.
When Banks withdrew that, he said: "What is clear now is that if we were to pursue a judicial review, according to legal experts, we would win.
"But this is a time to take a step back from the matter, and after consulting with leading campaigners on this issue, including Farage – we have decided to show the public how this process was stitched up, but not to pursue the judicial review any further."