Millionaire Butlins owner Peter Harris has been given the Electoral Commission's biggest financial penalty for breaking spending return rules during the EU referendum.
He was fined £12,000 for failing to submit returns for pro-Brexit adverts he ran during last year's poll.
Mr Harris spent more than £400,000 on newspaper ads featuring a British bulldog in a Union flag tie.
The commission said the ex-racehorse trainer has since paid the fine.
It said the £12,000 fine was "the highest imposed" in relation to spending at the EU referendum "and reflects the significant sums spent by Mr Harris".
The disclosure came as part of the independent watchdog's monthly investigations update and its commitment "to deliver transparency in political finance in the UK".
DUP 'duplicated payments'
Campaigners who spend more than £250,000 face penalties if they fail to deliver a spending return and audit report by the due date.
After he was advised of the commission's investigation, Mr Harris reported spending £421,432.64, and supplied an audit report.
The Democratic Unionist Party was also fined £4,000 for delivering "an inaccurate spending return" for the 2016 Northern Ireland Assembly election.
The return included payments to the value of £49,183.86 - which should not have been included as they were reportable by individual candidates in their own returns.
The commission said it was "satisfied, however that those payments had been properly reported" by the candidates, and their inclusion in the party spending return "was duplication".
The return was also missing two payments to the value of £397.50, which should have been reported.
The DUP has informed the commission that it has reviewed its procedures to prevent a recurrence of these errors.