The Brexit activist who has accused the Vote Leave campaign of exceeding legal spending limits said his claims will be backed up by evidence presented to MPs.
Shahmir Sanni alleged that the official Brexit campaign used a different group, BeLeave, to overspend.
Both groups deny any wrongdoing and Foreign Secretary Boris Johnson, a prominent Vote Leave campaigner, said the claims were “utterly ludicrous”.
Labour’s Tom Watson said that “if needs be” the police should investigate.
Mr Sanni said his lawyers would be presenting “recommendations to MPs” at a news conference in London on Monday.
He told the BBC: “It’s quite literally a matter of democracy and it’s up to the people of this country to have a look at the evidence themselves and decide for themselves whether the allegations are false or true.
“Lawyers are looking at the evidence and [have] said there is reasonable ground and so they are going to be presenting their recommendations to MPs.”
In interviews with Channel 4 News and the Observer, Mr Sanni – a volunteer for Vote Leave – said the official Brexit campaign donated £625,000 to a youth-oriented group, BeLeave, but required it to spend the money on a firm Vote Leave used for its digital advertising.
Vote Leave would have gone over its campaign spending limit of £7m if it had spent the money.
The Electoral Commission is already looking into whether Vote Leave spent more than it was entitled to during the campaign.
Vote Leave chief Dominic Cummings denied the claim and Brexit Secretary David Davis told the BBC it was the job of the commission, not ministers, to make a judgement.
Mr Sanni told the Observer £625,000 was given to the founder of BeLeave, Darren Grimes, who – he claims – was not in control of how the money was spent.
He said everything they did they passed through Vote Leave manager Stephen Parkinson, Mr Sanni’s ex-boyfriend, who is now the prime minister’s political secretary.
Mr Sanni told the newspaper that most of the donation went to Canadian data firm Aggregate IQ, which has been linked to Cambridge Analytica – the firm facing claims it amassed the data of millions of people without their consent.
He said he – along with two other pro-Brexit friends – reported the overspending allegation to the Electoral Commission on Thursday.
In effect they used BeLeave to overspend, and not just by a small amount… Almost two thirds of a million pounds makes all the difference and it wasn’t legal,” said Mr Sanni, who first worked as a Vote Leave outreach volunteer before working for BeLeave.
“They say that it wasn’t coordinated, but it was. And so the idea that… the campaign was legitimate is false.”
Vote Leave has previously said it made the donation to Mr Grimes because it was coming up to its £7m spending limit and wanted a way of using the £9.2m it had raised from individuals and companies on campaigning activities – and that it had received permission to from the Electoral Commission to do this.
The campaign separately spent £2.7m on the services of AIQ in the run-up to the EU referendum.
BeLeave was set up to give young pro-Brexit campaigners a voice during the referendum.
Separate campaign groups could spend up to £700,000 if they registered as permitted participants.
In a blog on Friday, Mr Cummings denied allegations of links between his campaign and Cambridge Analytica and said the claims were “factually wrong, hopelessly confused, or nonsensical”.
Lawyers for AIQ told Channel 4 News that it had “never entered into a contract with Cambridge Analytica” and it had “never knowingly been involved in any illegal activity”.
In a “personal statement” issued to Channel 4 News, Stephen Parkinson denied the allegations and said he was confident he had stayed within the law and spending rules “at all times”.
He said he was “saddened” by what he called “the factually incorrect and misleading” statements from Mr Sanni, who now works for the Taxpayers’ Alliance.
Earlier, Mr Sanni said – in a statement issued through his lawyers – that Mr Parkinson had outed him as gay in his original response.
Mr Sanni, a British Pakistani, said he was forced to tell his family and that relatives in Pakistan could be in danger as a result.
In his original statement, published on Mr Cummings’ blog on Friday, Mr Parkinson said he dated Mr Sanni for 18 months, before splitting up in September 2017.
“That is the capacity in which I gave Shahmir advice and encouragement, and I can understand if the lines became blurred for him, but I am clear that I did not direct the activities of any separate campaign groups,” he said.
Mr Grimes told Channel 4 News he denied the allegations.
A solicitor for Vote Leave told the programme the campaign had been cleared twice on this issue by the Electoral Commission.
The Electoral Commission said: “The commission has a number of investigations open in relation to campaigners at the EU referendum; it does not comment on live investigations.”