The elections watchdog is expected to find that official Brexit campaign Vote Leave broke spending rules during the 2016 referendum.
The campaign group’s former chief executive, Matthew Elliott, said the Electoral Commission (EC) had concluded that Vote Leave exceeded spending limits by making a donation to another Brexit-backing group.
The campaign, which had support from senior Tories including Boris Johnson and Michael Gove, is expected to challenge the commission’s findings when they are officially produced.
Mr Elliott accused the EC of a “huge breach of natural justice”, alleging that the watchdog had not listened to Vote Leave’s version of events.
If found guilty of breaking electoral law, the campaign could face a hefty fine and anti-Brexit organisations could seize on the findings to boost their cause.
Mr Elliott 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 EC said Vote Leave had taken an “unusual step” of going public with the findings of its draft report.
It is alleged the money was actually used for the benefit of Vote Leave, to pay data firm Aggregate IQ for targeted messaging services.
If this cash was recorded as Vote Leave expenditure, it would take the campaign’s spending over the £7 million limit, raising the prospect that electoral law had been breached.
The allegations come from information 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.
Mr Elliott told Sky News: “(The EC) 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.”
“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.”
Mr Gove, who was the campaign co-chairman during the referendum alongside Labour’s Gisela Stuart, said he had not seen the report but indicated that the EC’s findings could be challenged in the courts.
“The report itself, I think, is going to be challenged legally,” he told BBC Radio 4’s Today programme.
“If it’s going to be challenged legally, if it is going to go through the courts, it would be inappropriate for me – not having read the report – to offer a commentary on it.”
Pro-EU Labour MP Chuka Umunna said: “So, in an EU referendum where there was a 4% victory for Vote Leave, they overspent by 10%.
“We know they lied; we now know they cheated too and it’s official. Very serious implications.”
An EC 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.”
SNP Westminster leader Ian Blackford later raised the matter at Prime Minister’s Questions.
He said: “Does the Prime Minister agree that we need absolute transparency in elections and that people must be held accountable?”
Theresa May said she would not comment on “a leaked report”, but added: “The Electoral Commission, in relation to the Vote Leave matter, has said that it will consider representations it’s received and will publish a thorough and detailed closing report in order to provide a balanced account.
“We’ll of course consider that report when the Government receives it and will also consider any recommendations arising from it.”