Vote Leave has been fined and reported to the police by the Electoral Commission for breaking electoral law.
The Electoral Commission published the conclusions of its investigation into the campaign spending of Vote Leave and found “significant evidence” of coordination with another campaign group, BeLeave.
The Electoral Commission found that the campaign group exceeded its legal spending limit of £7m by almost £500,000 during the 2016 referendum campaign.
The watchdog found that Vote Leave, which was the official campaign for leaving the EU during the referendum, and Darren Grimes, the founder of BeLeave, broke electoral law.
A Vote Leave spokesman responded to the report and said, "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".
He accused the Commission, which said Vote Leave has resisted its investigation and committed serious breaches of the law, of being motivated by a political agenda and said the group was confident that the findings would be overturned.
The commission said BeLeave spent more than £675,000 with Aggregate IQ under a common plan with Vote Leave, which was not declared by the official Brexit campaign, leading to it exceeding its spending limit.
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, it added.
Grimes was found to have committed two offences and has been fined £20,000. The commission said Grimes spent more than £675,000 on behalf of BeLeave a non-registered campaigner that had a spending limit of £10,000. He also wrongly reported that same spending as his own, it added.
The watchdog has now referred both David Halsall, the responsible person for Vote Leave, and Grimes to the Metropolitan Police in relation to false declarations of campaign spending.
Bob Posner, the commission’s director of political finance and regulation and legal counsel, said: “The Electoral Commission has followed the evidence and conducted a thorough investigation into spending and campaigning carried out by Vote Leave and BeLeave.
“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.”
The breaches were “serious” Posner said, adding that parliament had put the laws in place to ensure fairness and transparency at elections and referendums.
Commenting on the investigation itself, Posner said: “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 cooperate, 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.”
The investigation also found that the campaign group Veterans for Britain inaccurately reported a donation it received from Vote Leave. It has been fined £250.
There was no evidence that Veterans for Britain campaigned under a common plan with Vote Leave.