Lead Brexit campaign group Vote Leave has been fined £61,000 and referred to the police for breaking electoral law, the Electoral Commission has announced.
Evidence shows pro-Brexit youth group BeLeave spent more than £675,000 with marketing firm Aggregate IQ under a common plan with Vote Leave, which should have been declared by Vote Leave, the commission said.
It means Vote Leave exceeded its legal spending limit of £7 million by almost £500,000, the statement added.
Bob Posner, the Electoral Commission's director of political finance and regulation and legal counsel, 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."
Mr Posner added: "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.
"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."