The founder of pro-Brexit campaign group BeLeave has begun his appeal against a £20,000 fine imposed by the Electoral Commission, citing “errors of fact, the law and unreasonableness” by the regulator as reasons for overturning the original decision.

Darren Grimes was fined in 2018 after being accused of breaching spending rules during the EU referendum campaign three years ago.

The 25-year-old insisted he was “completely innocent” of making false declarations in relation to a £680,000 donation to his youth-focused BeLeave group 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.

Mr Grimes, a former fashion student originally from County Durham, raised £93,956 via an online crowdfunding campaign to appeal against the verdict of the commission, which began at the Mayor’s and City of London Court in Central London on Monday.

Tim Straker QC, representing Mr Grimes, told Judge Marc Dight the Electoral Commission made “a sequence of errors”, including some related to record-keeping.

He told the 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.”

He said the regulator had “played a part in vilifying” his client, who sat behind him in court throughout the hearing.

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 ahead of the court appearance, Mr Grimes said the Electoral Commission “completely ruined my early 20s” and said he faced bankruptcy or the prospect of his family selling their home if the decision went against him.

The EU referendum in 2016 saw 52% of the votes cast in favour of leaving the bloc, triggering the resignation of then-prime minister David Cameron.