The British Electoral Commission on Tuesday referred the official group behind the campaign for the UK to leave the EU to police for campaign fraud.
The commission said the Vote Leave had worked with another campaign group, BeLeave, without disclosing the relationship. The BeLeave campaign spent £675,000 (Dh3.29m) with Aggregate IQ, a group that used social media to target voters.
"We found substantial evidence that the two groups worked to a common plan, did not declare their joint working and did not adhere to the legal spending limits," said Bob Posner, the commission's director of political finance and regulation.
Mr Posner added that Vote Leave had resisted the investigation, and refused to cooperate.
"Nevertheless, the evidence we have found is clear and substantial," Mr Posner said. "These are serious breaches of the laws put in place by Parliament to ensure fairness and transparency at elections and referendums."
Vote Leave, the officially designated campaign arguing the case for Brexit, was also fined £61,000. The group referred David Halsall, the responsible person for Vote Leave, and Darren Grimes, a fashion student who founded the BeLeave campaign group, to police for false declarations of campaign spending.
Vote Leave denied the accusations, arguing in a statement that they were politically motivated.
"The Electoral Commission’s report contains a number of false accusations and incorrect assertions that are wholly inaccurate and do not stand up to scrutiny," a Vote Leave spokesman said in a statement.
"All this suggests that the supposedly impartial Commission is motivated by a political agenda rather than uncovering the facts."
Under the rules governing the referendum on June 23, 2017, the campaigns had strict spending limits. By working with a separate group without disclosing the relationship, Vote Leave could have been able to coordinate a drive that outspent the rival group campaigning to remain.