Brexit campaign group Vote Leave has been fined and referred to the police for breaking electoral law, the Electoral Commission has announced.
The move follows a probe into campaign spending.
The commission said its investigation found “significant evidence” of joint working between lead campaigner, Vote Leave, and another campaign group, BeLeave.
Vote Leave accused the commission of being “motivated by a political agenda rather than uncovering the facts”.
The commission said: “Evidence shows that BeLeave spent more than £675,000 with Aggregate IQ under a common plan with Vote Leave.
“This spending should have been declared by Vote Leave.
“It means Vote Leave exceeded its legal spending limit of £7 million by almost £500,000.
“Vote Leave also returned an incomplete and inaccurate spending report, with nearly £234,501 reported incorrectly, and invoices missing for £12,849.99 of spending.
"Darren Grimes, the founder of the BeLeave campaign group, was found to have committed two offences and has been fined £20,000.
“Mr Grimes spent more than £675,000 on behalf of BeLeave, a non-registered campaigner that had a spending limit of £10,000.
“Further, he wrongly reported that same spending as his own.
"The commission has now referred both Mr David Halsall, the responsible person for Vote Leave, and Mr Grimes to the Metropolitan Police in relation to false declarations of campaign spending."
The commission imposed fines of £61,000 on Vote Leave.
Bob Posner, Electoral Commission director of political finance, said: "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.
“These are serious breaches of the laws put in place by Parliament to ensure fairness and transparency at elections and referendums.
“Our findings relate primarily to the organisation which put itself forward as fit to be the designated campaigner for the ‘leave’ outcome.”
Mr Posner added: “Vote Leave has resisted our investigation from the start, including contesting our right as the statutory regulator to open the investigation.
“It has refused to co-operate, refused our requests to put forward a representative for interview, and forced us to use our legal powers to compel it to provide evidence.
“Nevertheless, the evidence we have found is clear and substantial, and can now be seen in our report.”
A Vote Leave spokesman said the report contained “a number of false accusations and incorrect assertions that are wholly inaccurate and do not stand up to scrutiny”.
He said: “Vote Leave has provided evidence to the Electoral Commission proving there was no wrongdoing. And yet, despite clear evidence of wrongdoing by the Remain campaign, the commission has chosen to ignore this and refused to launch an investigation.
“All this suggests that the supposedly impartial commission is motivated by a political agenda rather than uncovering the facts.
“The commission has failed to follow due process, and in doing so has based its conclusions on unfounded claims and conspiracy theories.
“We will consider the options available to us, but are confident that these findings will be overturned.” – PA