Brexit campaigners breached UK law to channel more funds to a data analysis firm ahead of the EU referendum, according to evidence outlined on Monday by whistleblowers' lawyers.
"There is a prima facie case that the following electoral offences were committed by Vote Leave in the EU referendum campaign," said a 50-page document presented by London law firm Bindmans.
The case centres on a £625,000 ($889,300, 714,150 euros) donation by Vote Leave to the smaller pro-Brexit group BeLeave, which lawyers argue was made by the lead campaign group to mask a payment to Canadian data firm AIQ.
"There are strong grounds to infer that Vote Leave was involved in the decision by which the AIQ payment were made," said the legal opinion, written by three independent barristers at Bindmans' request.
Shahmir Sanni, who worked for BeLeave, earlier told British media of the campaign's close ties to Vote Leave, "in effect they used BeLeave to over-spend, and not just by a small amount".
"They say that it wasn't coordinated, but it was. And so the idea that... the campaign was legitimate is false," he told Channel 4 News.
Vote Leave ultimately declared campaign costs of just over £6.7 million -- under the £7 million legal limit -- of which nearly £2.7 million was spent on services by AIQ.
Lawyers argue witness statements by three people close to the two campaigns demonstrate their "extremely close relationship", including staff from the two groups sharing an office.
The case requires "urgent investigation" to decide whether to refer it to state prosecutors, they said.
The overspending accusation comes as the role of data analysis companies in political campaigns come under greater scrutiny, following allegations that British consulting group Cambridge Analytica harvested data on tens of millions of Facebook users.
Christopher Wylie, who worked for Cambridge Analytica, blew the whistle on the scandal and appeared at the Bindmans press conference where he said there should be a second Brexit referendum.
"This is about the integrity of the fundamental process... It is important that we enforce the law where over-spending happened," he said, declaring himself a eurosceptic.
The legal opinion said top Vote Leave figures "must have known about BeLeave's campaign activity", including Stephen Parkinson who is now Prime Minister Theresa May's political secretary.
Vote Leave has denied the accusations and is backed by British foreign minister Boris Johnson, a Brexit figurehead, who on Sunday dismissed the claims as "utterly ludicrous".