Vote Leave has been fined £61,000 and reported to the police by the Electoral Commission after the watchdog found “significant evidence” of coordination with another campaign group, BeLeave.

The watchdog said it had imposed punitive fines on Vote Leave because it said the group had refused to cooperate fully with its investigation and declined to be interviewed. Its former chief executive, Matthew Elliott, had previously alleged it was the Electoral Commission that had refused to cooperate. Vote Leave called the findings “wholly inaccurate”.

The commission’s long-awaited report said it had found evidence BeLeave spent more than £675,000 with the digital data company Aggregate IQ coordinated with Vote Leave, which should have been declared by the Brexit campaign group.

Vote Leave, which was the official designated campaign for Britain to leave the EU during the referendum, fronted by Boris Johnson and Michael Gove, exceeded its legal spending limit of £7m by almost £500,000, the watchdog found.

Darren Grimes, the founder of BeLeave, and the Vote Leave official David Halsall have been reported to the police. Vote Leave has been fined £61,000 and Grimes £20,000.

The commission said it had shared its investigation files with the Metropolitan police to investigate whether any other offences had been committed outside the watchdog’s remit.

The commission’s full report said Vote Leave repeatedly refused to attend interviews. The watchdog said:

Interviews were requested in November 2017, when Vote Leave indicated it could cooperate.

Vote Leave did not respond to a request to set interview dates in December and January 2018.

Vote Leave then sent legal letters to the Electoral Commission threatening to judicially review the opening of the investigation.

The commission made two further offers of interview dates but said Vote Leave “began to repeat procedural questions we had already answered”.

Vote Leave was then issued with a formal investigation notice to provide certain documents.

The group did not reply by the deadline or produce the documents but said they could be inspected at its lawyer’s office.

Vote Leave then made its offer of inspection of the documents contingent on a meeting to discuss why the investigation should be closed, which the commission said was not “appropriate or helpful”.

The documents made available were then found to be incorrect or incomplete.

The Electoral Commission chief executive, Claire Bassett, was scathing about Vote Leave’s refusal to cooperate, telling BBC Radio 4’s Today programme: “Over a three-month period we actually made five attempts to interview Vote Leave and we were unable to. We have in fact issued a record fine for failure to cooperate with a statutory notice because we found it so difficult to get Vote Leave to work with us in this investigation.”

Bob Posner, of the Electoral Commission, also criticised the campaign, saying Vote Leave had “resisted our investigation from the start, including contesting our right as the statutory regulator to open the investigation”.

“It has refused to cooperate, 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,” hesaid.

Posner said the commission had found “serious breaches of the laws put in place by parliament to ensure fairness and transparency at elections and referendums”.

Elliott, now the editor-at-large of the website Brexit Central, tweeted: “Having skimmed the EC’s [Electoral Commission’s] report, they’ve ignored VL’s [Vote Leave’s] detailed evidence, so it’s riddled with errors & conclusions completely wrong. We accepted their invitation for an interview in early March. Senior staff also volunteered to be interviewed. They haven’t followed due process.”

Elliott had revealed the outcome of the investigation would be damaging to Vote Leave in interviews before the report was officially released. He said Vote Leave was contesting the claims and accused the watchdog of “a highly political agenda”.

He said his group followed “the letter of the law and spirit of the law” and alleged that the commission had not interviewed senior figures from Vote Leave. He told the BBC he had submitted a 500-page dossier to the Electoral Commission rebutting the claims.

The investigation also found the campaign group Veterans for Britain inaccurately reported a donation it received from Vote Leave. It has been fined £250. The watchdog found no evidence that Veterans for Britain campaigned under a common plan with Vote Leave.

A Vote Leave spokesman said the report “contains a number of false accusations and incorrect assertions that are wholly inaccurate and do not stand up to scrutiny”. It said Vote Leave had repeatedly made it clear it was willing to be interviewed.

“Yet the commission has interviewed the so-called ‘whistleblowers’ who have no knowledge of how Vote Leave operated and whose credibility has been seriously called into question,” it continued.

“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.”

Theresa May’s spokesman said: “The PM is absolutely clear that this was the largest democratic exercise in our country. The public delivered a clear verdict and that is what we are going to be implementing.”

May is expected to come under pressure in relation to two Downing Street advisers, Stephen Parkinson and Cleo Watson, both of whom had senior roles in Vote Leave. Downing Street said the prime minister had full confidence in both.