The founder of pro-Brexit campaign group BeLeave has won his appeal against a £20,000 fine imposed by the Electoral Commission.
Darren Grimes was fined in 2018 after being accused of breaching spending rules during the EU referendum campaign three years ago.
But 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 at the Mayor's and City of London Court in Central London.
The commission had misinterpreted the law and set a key legal test "too high" on whether BeLeave had been correctly registered on official forms, the court heard.
Mr Grimes had said that he had intended to register the organisation BeLeave and not himself as an individual on the forms.
His lawyers said he had filled out the complex and difficult-to-understand forms to the best of his ability.
Even if the group BeLeave did not have a formal constitution by January 2016, it was clear it was made up of like-minded people who had an agreement to campaign over the Brexit issue in a certain way, according to Judge Marc Dight.
The judge said Mr Grimes had tried to meet his obligations to the commission in filling out the forms, and that his actions were not dishonest or lacking transparency.
Following the ruling Mr Grimes said: "I am delighted and relieved that the court has found me innocent.
"This case has taken a huge toll on myself and my family, and I'm thankful it's now over. I will be eternally grateful to all those people who have supported me - your generosity and kind words of encouragement have kept me going."
In the message, posted on Twitter, he hit out at the watchdog's handling of the case.
"The Electoral Commission's case was based on an incorrectly ticked box on an application form - something that it had been aware of for over two years and had not been raised in two previous investigations.
"Yet the Commission still saw fit to issue an excessive fine and to spend almost half a million in taxpayer cash pursuing me through the courts. This raises serious questions about its conduct both during and after the referendum.
"It's vital that more young people are encouraged to get involved in politics and make their voices heard. I just hope that the punitive actions of the Electoral Commission don't put my generation off from engaging in our democracy."
The Electoral Commission said: "We are disappointed that the court has upheld Mr Grimes' appeal.
"We will now review the full detail of the judgment before deciding on next steps, including any appeal."
The effect of Judge Marc Dight's ruling is that the order against Mr Grimes is quashed and the commission will have to pay his costs, according to Mr Grimes's lawyer, Timothy Straker QC.
The amount of the costs still has to be assessed.
Tory MP Charlie Elphicke said it was now clear there should be "big changes at the Electoral Commission".
Matthew Elliott, who was Vote Leave's chief executive, said a movie should be made about Mr Grimes' battle with the Electoral Commission.
"He has shown amazing courage and fortitude over the past three years and is truly a hero of the Brexit cause," Mr Elliott said.