Information provided by whistleblowers provides grounds to suspect that the UK's Vote Leave campaign broke Britain's electoral law during the 2016 EU referendum campaign, according to a legal opinion obtained by the men's lawyers.
And the opinion said there were "reasonable grounds" for the UK's Electoral Commission to investigate the possibility of a conspiracy involving two senior members of the Leave campaign now working as advisers to Theresa May in 10 Downing Street.
The 50-page opinion obtained by Bindman's solicitors, acting on behalf of Christopher Wylie and Shahmir Sanni, called for an "urgent investigation" to establish whether a prosecution could be brought over allegations the campaign broke spending limits.
The allegations have all been denied by Vote Leave and its former officials, who reject all accusations of wrongdoing.
Witness statements and documents provided by the whistleblowers "strongly suggest" that a donation of almost £680,000 made by the campaign to a youth Brexit group called BeLeave was actually used for
the benefit of Vote Leave, to pay data firm Aggregate IQ for targeted messaging services, said the legal opinion, prepared by barristers from Matrix Chambers.
If this cash was recorded as Vote Leave expenditure, it would take the campaign's spending over the £7m limit, establishing a "prima facie case" that electoral law has been breached.
There are "realistic prospects" that the group and official David Halsall might be convicted, said QCs Clare Montgomery and Helen Mountfield and barrister Ben Silverstone.
And they said there were "reasonable grounds" for the Electoral Commission to investigate whether any offences where committed "with the knowledge, assistance and agreement" of senior figures in Vote Leave, including Stephen Parkinson and Cleo Watson, who are now advisers to the British Prime Minister, as well as the campaign director Dominic Cummings.
"Given the very close working relationships at all material times between Vote Leave and BeLeave, the way in which Mr Parkinson and Ms Watson supervised the work of the young BeLeave volunteers and that Vote Leave and BeLeave staff worked closely together on a daily basis, in the same office, throughout the referendum campaign, it can be properly inferred that Mr Parkinson and Mr Watson must have known about BeLeave's campaign activity, of which the AIQ targeted messaging was a significant part," said the opinion.
"In these circumstances, there are certainly reasonable grounds for the Commission to use its powers... to investigate whether any election offences committed by Vote Leave and Mr Halsall were committed with the knowledge, assistance and agreement of other senior figures/officers in Vote Leave, including Mr Parkinson and Ms Watson."
Bindmans partner Tamsin Allen said the information upon which the legal opinion was based had been passed to the Electoral Commission, which did not have access to it at the time of its earlier inquiry into Vote Leave.
She said no communication had been obtained proving that Vote Leave ordered BeLeave to pass money to AIQ but there was a "strong inference" that this had happened.
Ms Allen said the solicitors were funded by anonymous donors to assist Mr Wylie at the time of his initial revelations regarding data collection by Cambridge Analytica, but this money had now run out.
"He was facing really serious personal and legal risk," she said. "I started working for him unpaid, then several donors were found.
"Given the risk to Chris and the risk that these people felt too, I have agreed to keep their names confidential and I will respect that confidentiality obligation.
"That money is now exhausted. Both I and counsel have done a significant amount of unpaid work and we are continuing to do so."
Cash was being raised via the crowdfunding site crowdjustice.com, she said.
Mr Wylie said he believed Mr Parkinson and Ms Watson should lose their jobs at Downing Street.
It was "absolutely outrageous" that a Number 10 official had passed on to a New York Times journalist a copy of a blog by Mr Cummings in which Mr Parkinson effectively outed Mr Sanni as gay, he said.
Ms Allen said Bindmans had successfully requested that references to Mr Sanni's sexuality should be removed from the blog and believed the issue had been "contained", only to find it had been sent out by the Downing Street official.
"At that point, it was clear to us that there was no containing that information anymore," she said.
"That email, to us, meant that he had effectively been outed in a statement from an official Downing Street email."
Mr Wylie said that if wrongdoing was proved, the referendum should be re-run.
"If you are caught doping in the Olympics, you get the medal taken away," he said.
"This is about the integrity of the democratic process and can we trust a result where it is highly likely that overspending happened.
"British democracy is about listening to the voice of the people, not how much money you can spend, so it's important that we enforce the law where overspending happened."