The Electoral Commission has fined the Brexit campaign group Vote Leave £61,000.
The watchdog imposed the fine after finding “significant evidence of joint working” with the organisation BeLeave, which led to exceeding the spending limit by almost £500,000.
The Vote Leave campaign also refused to fully co-operate fully with the investigation and did not participate in interviews, said Electoral Commission chief executive Claire Bassett.
“Over a three-month period we actually made five attempts to interview Vote Leave and we were unable to,” she told BBC Radio 4’s Today programme.
“We have in fact issued a record fine for failure to cooperate with a statutory notice because we found it so difficult to get Vote Leave to work with us in this investigation.”
According to the report, the group had invoices missing for £12,849.99 of its spending.
“The Electoral Commission has followed the evidence and conducted a thorough investigation into spending and campaigning carried out by Vote Leave and BeLeave,” said Bob Posner, from the Electoral Commission.
“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.”
“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,” he added.
“Nevertheless, the evidence we have found is clear and substantial, and can now be seen in our report.”
Vote Leave was the official campaign for the 2016 EU referendum and was backed by Brexiteers Boris Johnson and Michael Gove.