Leave.EU, one of the biggest groups campaigning to leave the European Union ahead of the UK referendum, has been fined £70,000 (€80,000) by the elections watchdog.
The Electoral Commission said the campaign had spent at least 10% more than it was allowed and had not declared its funding correctly.
One of the group's officials has been referred to police after the investigation found "reasonable grounds" to suspect criminal offences had been committed
The campaign, co-founded by Eurosceptic businessman Arron Banks, was heavily involved in the campaign despite losing out in a bid to gain official status to rival Vote Leave.
Banks told the Guardian that the Electoral Commission's findings were a "politically motivated attack" on Brexit and that he would challenge them in court.
The investigation found no evidence to support claims of a link between the campaign and Cambridge Analytica, the firm which used unauthorised data to target voters in the US election.