The UK's election watchdog is expected to find that Vote Leave, the official Brexit campaign, breached electoral rules during the referendum.
The campaign group's former chief executive, Matthew Elliott, said the Electoral Commission had concluded that Vote Leave exceeded spending limits and made a donation to a group that it should not have.
Mr Elliot went public with the draft findings, which have not been published, late on Tuesday night. The Electoral Commission said Vote Leave had taken an "unusual step" of going public with the findings of its draft report.
If found guilty of breaking electoral law, the campaign could be fined. Allegations against the official Brexit campaign centre on a donation of almost £680,000 made by the campaign to a youth Brexit group called BeLeave.
READ MORE: Paris Gourtsoyannis: Media is doing its job over Brexit ‘dark money’
It is alleged that the two groups coordinated on strategy, and that money was used to pay data firm Aggregate IQ for targeted messaging services that benefited Vote Leave. If the funds had been recorded as Vote Leave expenditure, it would have taken the campaign's spending over a £7 million limit, breaching electoral law.
The Electoral Commission has investigated the allegations twice before. However, new information was provided by whistleblowers including Christopher Wylie and Shahmir Sanni. Mr Wylie worked for Cambridge Analytica, the data firm at the centre of the Facebook privacy scandal, while Mr Sanni worked with Vote Leave.
According to the BBC, Vote Leave now admit that there was email correspondence between the donor who made the donation to BeLeave and Dominic Cummings, the Vote Leave director.
Mr Cummings is facing a rare charge of being in contempt of parliament for refusing to appear before a Commons committee holding an inquiry into 'fake news'.
Mr Elliot accused the EC of a "huge breach of natural justice", alleging that the watchdog had not listened to Vote Leave's version of events.
He told Sky News: "Their initial conclusion is that we have overspent, that a donation we made to another group during the course of the campaign was incorrect, we shouldn't have made that donation."
"[The Electoral Commission] listened to these, quite frankly marginal characters who came out in March, and listened to their stories, but haven't had evidence from Vote Leave side of things.
"I think it is a huge breach of natural justice that they haven't wanted to listen to our opinions and our story and we were the people running the campaign."
In an interview with the BBC he said: "I believe we acted both within the letter of the law and also the spirit of the law and the spirit of how you should conduct a campaign.
"We got the designation, Vote Leave, as the officially designated campaign for Leave, on the basis that we would be working with other groups - we wouldn't just solely be working on our own, we would work alongside other groups and encourage them, and encourage their activities."
READ MORE: SNP MP: ‘Dismiss Scottish people’s voice on Brexit at your peril’
An Electoral Commission spokesman said: "The commission has concluded its investigation and having reached initial findings provided Vote Leave with a 28-day period to make any further or new representations. That period ended on Tuesday 3 July.
"The unusual step taken by Vote Leave in sharing its views on the Electoral Commission's initial findings does not affect the process set out in law.
"The commission will give due consideration to the representations made to the commission, including those made by other campaigners under investigation.
"We will then, at the earliest opportunity, publish a thorough and detailed closing report in order to provide a full and balanced account to the public and to Parliament."