Lawyers acting for whistleblowers have released further evidence they say shows the Vote Leave campaign broke EU referendum spending rules.
The material allegedly shows how closely the campaign worked with youth group BeLeave.
It comes from Mark Gettleson, a web designer, the third person to make claims about Vote Leave’s spending in evidence to a select committee.
Vote Leave has rejected claims of illegal co-ordination with BeLeave.
Legal Firm Matrix Chambers argues, in a 50-page legal opinion, that Vote Leave should have declared payments of just over £625,000 to Canadian data firm, AIQ.
If included in the campaign group’s overall spending return, the payments would have pushed Vote Leave over the £7m limit.
Vote Leave says the £625,000 doesn’t count as its own expenditure, as the money was a donation to Darren Grimes, who set up the group, BeLeave.
He says he spent the money on services provided by AIQ, although the money went directly to AIQ from Vote Leave, for “services in kind” to BeLeave.
The Electoral Commission has said this would have been within the rules, provided that Vote Leave and BeLeave were not working together – a decision that is the subject of a separate legal challenge by the Good Law Project.
Former Vote Leave activist Shahmir Sanni and Christopher Wylie, who worked for controversial data firm Cambridge Analytica, have already claimed that Vote Leave used BeLeave to get round spending limits.
Foreign Secretary Boris Johnson, a key figure in the Vote Leave campaign, has claimed that the allegations are “ludicrous” and that the Leave campaign won the poll “fair and square and legally”.
In the Matrix Chambers legal opinion, Mark Gettleson – who is referred to as J – adds further details of alleged working together.
It also reveals that he met Darren Grimes when they were both working on MP Norman Lamb’s Lib Dem leadership campaign.
The Fair Vote group, which campaigns for another EU referendum, has published some of Mr Gettleson’s evidence on its website.
It includes an email from Mr Gettleson, who worked for Vote Leave between February and April 2016, that says that Vote Leave was responsible for the BeLeave campaign concept and website.
It also shows Mr Gettleson introduced Vote Leave to AIQ, saying he “headhunted the successful digital team,” the campaign group says.
Another email between Mr Gettleson and Cleo Watson, Vote Leave’s head of outreach, is about another group, Veterans for Britain, which donated £100,000 to AIQ. Campaigners claim Veterans for Britain was also used by Vote Leave to sidestep the rules.
Kyle Taylor, director of the Fair Vote Project, said: “This new evidence, on top of Shahmir Sanni’s and Chris Wylie’s, should without a doubt force Parliament to take immediate action and show the British public that it cares about protecting one of our highest ideals – democracy.”
Mr Gettleson’s lawyer, Tamsin Allen, of Bindmans solicitors, said he had hoped to remain anonymous but realised the details in the legal opinion might have led to him being identified and had taken the decision to release the emails.
The legal opinion was submitted by Matrix Chambers as evidence to the Digital, Culture, Media and Sport Select Committee’s inquiry into fake news.
“We consider that there is a prima facie case that… electoral offences were committed by Vote Leave in the EU referendum campaign,” the opinion says.
The Electoral Commission, which is investigating Vote Leave’s spending, declined to comment.
Former Vote Leave officials and Veterans for Britain have been contacted for a response.