In the referendum, the official pro- and anti-Brexit campaign groups, designated by the Electoral Commission, had spending limits of £7 million (US$9.3 million, €7.9 million).
Vote Leave was fined $106,065 and the Electoral Commission referred David Halsall, the responsible person for Vote Leave, and Darren Grimes, the founder of the BeLeave campaign group, to the police for false declarations of campaign spending.
The Electoral Commission's findings come as Prime Minister Theresa May's Conservative party remains starkly divided over if and how the United Kingdom should split from the European Union.
The commission handed Vote Leave a record £61,000 fine and passed the findings to the police for a possible criminal investigation.
This should have been declared by Vote Leave, the commission said.
The commission said it is "satisfied beyond reasonable doubt" that payments totalling 675,000 British pound (about $894,000) by BeLeave to data analytics firm AggregateIQ were made "under a common plan with Vote Leave".
'These are serious breaches of the laws put in place by Parliament to ensure fairness and transparency at elections and referendums.
It found that Vote Leave also returned an incomplete and inaccurate spending report, with almost £234,501 reported incorrectly, and invoices missing for £12,849.99 of spending.More news: Trapped boys thought they were hallucinating when rescuers arrived
More news: Elon Musk apologises to British caver for ‘pedo’ slur
More news: Anderson Cooper calls Trump-Putin press conference ‘disgraceful’
FILE PHOTO: Head of Vote Leave, Matthew Elliott, poses for a photograph at the Vote Leave campaign headquarters in London, Britain May 19, 2016. Vote Leave declined to be interviewed.
Posner added that Vote Leave had resisted the investigation from the start and had refused to co-operate.
In a statement posted on Twitter, Grimes said he is "shocked and disappointed by the Electoral Commission and their behaviour".
Earlier this month, the former head of the Vote Leave campaign went public with the unpublished interim findings of the Electoral Commission's investigation, accusing the watchdog of not listening to their account.
The comments were made during a debate sparked by Labour MP Chuka Umunna - who is now campaigning to overturn Brexit - after he was granted an urgent question in the House of Commons after the news emerged.
Sharing a post by Guardian journalist Carole Cadwalladr which noted that Boris Johnson, Michael Gove and Theresa May's political secretary, Stephen Parkinson, were all heavily involved in Vote Leave, Russell added: "Important point".
"We can not say with confidence that this foul play did not impact on the result".
Senior European Union officials have said publicly that should Britain change its mind, before or after the March 2019 deadline, they would be happy to welcome the Brits back into the fold.