The founder of the youth-focused pro-Brexit campaign group BeLeave has won his appeal against a £20,000 fine imposed by the elections regulator.
Darren Grimes was punished by the Electoral Commission last year, after being accused of breaching spending rules during the 2016 EU Referendum.
Mr Grimes said he was "relieved" as the case had "taken a huge toll".
He had maintained that he was "completely innocent" of making false declarations.
The Electoral Commission had taken action in relation to a £675,315 donation from Vote Leave, but Mr Grimes accused it of "bias" against Leave supporters.
But the watchdog insisted its investigation had been "thorough and fair" and that it had carried out inquiries into campaigners on both sides of the referendum battle.
Last year, the Electoral Commission found that BeLeave had "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 £7m legal spending limit by almost £500,000.
But Mr Grimes, aged 25 and from County Durham, appealed against the fine after raising the money for legal costs online, citing "errors of fact, the law and unreasonableness".
The court heard that the commission had misinterpreted the law and set a key legal test "too high" on whether BeLeave had been correctly registered on official forms.
Mr Grimes had said he had intended to register the organisation and not himself as an individual on the forms and his lawyers said the complex and difficult-to-understand forms were completed to the best of his ability.
Judge Marc Dight said that even if 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 on Brexit in a certain way.
He 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.
Responding to the success of his appeal, at the Mayor's and City of London Court, Mr Grimes tweeted that he was "delighted and relieved", adding that the case had "taken a huge toll on myself and my family".
In his statement, he said: "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 engaging in our democracy."
He also criticised 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," his statement said.
"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."
The Electoral Commission said it was "disappointed".
"We will now review the full detail of the judgment before deciding on next steps, including any appeal," it said.