The founder of pro-Brexit campaign group BeLeave has begun an appeal against a £20,000 fine imposed by the Electoral Commission.
Twenty-five-year-old Darren Grimes was handed the fine in 2018 after being accused of breaching spending rules during the EU referendum campaign three years ago.
He says the fine "completely ruined my early 20s" and crowdfunded £93,956 online to appeal the Commission's verdict.
Mr Grimes maintains he is "completely innocent" of making false declarations in relation to a £680,000 donation to BeLeave from the main Vote Leave campaign.
The Electoral Commission - which regulates political parties, members and campaigners - found that BeLeave "spent more than £675,000 with (Canadian data firm) Aggregate IQ under a common plan with Vote Leave", which should have been declared by the latter but was not.
This spending took Vote Leave over its £7 million legal spending limit by almost £500,000.
Representing Mr Grimes, Tim Straker QC told Judge Marc Dight the Electoral Commission made "a sequence of errors", including some related to record-keeping.
He told Mayor's and City of London Court: "Can mistakes by a regulator put a man in receipt of a penalty when he behaved openly and transparently throughout? It cannot conceivably be right."
The appeal case is listed to last for up to 10 days, with judgment expected to be handed down at a later date.
In a video message to his supporters, Mr Grimes, a former fashion student, said he faced bankruptcy or the prospect of his family selling their home if the decision went against him.