The row centres around Darren Grimes, at the time a fashion student at the University of Brighton, who set up a group called BeLeave, to give young pro-Brexit campaigners a voice during last year's referendum.
As a registered campaigner, he was allowed to spend up to £700,000. He initially spent very little but in the 10 days leading up to the 23 June vote he ran up a £675,315 bill with AggregateIQ Data, a Canadian marketing firm that specialises in political campaigns.
ETA:
the official statement from the Electoral Commission.
Money to clear the bill was not given to Mr Grimes but sent directly to Aggregate IQ by Vote Leave, which separately spent £2.7m with the same firm, more than a third of its £6.8m budget.