Report by the Electoral Commission says the official Brexit campaign exceeded its legal spending limit by almost £500,000.
The official Brexit campaign, Vote Leave, has been fined and referred to the police for breaking electoral law.
The announcement follows an investigation by Channel 4 News and the Observer into allegations of overspending.
A report by the Electoral Commission said there was “significant evidence” of joint working between Vote Leave and other Brexit campaigners – BeLeave and its founder, Darren Grimes.
A donation of almost £680,000 – made by Vote Leave to BeLeave – took the official campaign’s spending over the legal limit of £7 million.
Some of the money was used to pay Aggregate IQ, the Canadian data firm that Channel 4 News also investigated in the wake of the Cambridge Analytica scandal.
The Electoral Commission said: “Evidence shows that BeLeave spent more than £675,000 with Aggregate IQ under a common plan with Vote Leave. This spending should have been declared by Vote Leave.
“It means Vote Leave exceeded its legal spending limit of £7 million by almost £500,000.”
Vote Leave, which was supported by the likes of Boris Johnson and Michael Gove, has now been fined £61,000.
Darren Grimes has also been fined £20,000 and referred to the Metropolitan Police, along with David Halsall, who was the responsible person for Vote Leave, “in relation to false declarations of campaign spending”, the Commission added.
In turn, Vote Leave accused the Commission of being “motivated by a political agenda rather than uncovering the facts”.
But Bob Posner, Electoral Commission director of political finance, said: “We found substantial evidence that the two groups worked to a common plan, did not declare their joint working and did not adhere to the legal spending limits.
“These are serious breaches of the laws put in place by Parliament to ensure fairness and transparency at elections and referendums.
“Our findings relate primarily to the organisation which put itself forward as fit to be the designated campaigner for the ‘leave’ outcome.”
The original allegations, broadcast by Channel 4 News, were made by whistleblowers including Shahmir Sanni, who helped run the BeLeave offshoot campaign with Darren Grimes. He claimed the money was used to pay Aggregate IQ for targeted messaging services on Facebook and other social media.
A Vote Leave spokesman said the Electoral Commission’s report contained “a number of false accusations and incorrect assertions that are wholly inaccurate and do not stand up to scrutiny”.
He said: “Vote Leave has provided evidence to the Electoral Commission proving there was no wrongdoing.
“And yet, despite clear evidence of wrongdoing by the Remain campaign, the Commission has chosen to ignore this and refused to launch an investigation.
“All this suggests that the supposedly impartial Commission is motivated by a political agenda rather than uncovering the facts.
“The Commission has failed to follow due process, and in doing so has based its conclusions on unfounded claims and conspiracy theories.
“We will consider the options available to us, but are confident that these findings will be overturned.”
He also reiterated the claim that the Commission failed to interview anyone from the campaign despite them being “willing to do so”.
In a statement posted on Twitter, Darren Grimes said: “It is incredible that the Electoral Commission has issued a £20,000 fine on the basis of the wrong box being ticked on an application form.
“The Commission has been aware of this for over two years, and had not raised the issue before in our extensive correspondence, during voluntary interview or its two previous investigations.
“This raises serious questions about why the Commission has taken these steps now despite no new evidence being provided. The fine is entirely disproportionate and unjustified.
“I also find it remarkable and surprising that the Commission has concluded I was part of a common plan with Vote Leave, despite never providing me with the particulars of this allegation so that I could make representations about it.
“Politicians say they want young people to engage with politics. I was 22 when I got involved in a referendum I felt passionately about. I did nothing wrong. I have been persecuted for over two years by powerful people for nothing more than engaging in the democratic process and having the temerity to be on the winning side. It has been appalling for my family.”
But Bob Posner from the Electoral Commission said: “Vote Leave has resisted our investigation from the start, including contesting our right as the statutory regulator to open the investigation.
“It has refused to co-operate, refused our requests to put forward a representative for interview, and forced us to use our legal powers to compel it to provide evidence.
“Nevertheless, the evidence we have found is clear and substantial, and can now be seen in our report.”
Labour MP David Lammy, part of the Best For Britain campaign, said: “This news makes the narrow referendum result look dodgier than ever. Its validity is now in question.
Vote Leave is the latest Brexit campaign group to be penalised over its spending during the EU referendum. In May, Brexit campaign group Leave.EU was fined £70,000 and its chief executive Liz Bilney referred to police.