Leave.EU has been fined £70,000 after an Electoral Commission investigation found it exceeded its spending limit by 10%.
It also incorrectly reported its spending during the referendum campaign and delivered inaccurate spending and transaction returns.
Leave.EU had been linked with the now-defunct Cambridge Analytica, but the investigation found no evidence that it received donations or paid-for services from Cambridge Analytica.
The relationship, it said, "did not develop beyond initial scoping work".
The investigation did find, though, that services received from the US campaign strategists Goddard Gunster were not included in the spending return "despite a proportion of them having been used during Leave.EU's referendum campaign".
The commission also found that Leave.EU, co-founded by millionaire Brexit backer Arron Banks, inaccurately reported three loans it had received.
It added: "This included a lack of transparency and incorrect reporting around who provided the loans, the dates the loans were entered into, the repayment date and the interest rate.
"Finally, Leave.EU failed to provide the required invoice or receipt for 97 payments of over £200, totalling £80,224. "
Bob Posner, the Electoral Commission's director of political finance and regulation and legal counsel, described the breaches as "serious offences" and said it was disappointing that a key player in the referendum campaign had broken the rules.
He also suggested that the fine would have been bigger but for a cap on the amount the commission can levy.
"The rules we enforce were put in place by Parliament to ensure transparency and public confidence in our democratic processes,” he said.
"It is therefore disappointing that Leave.EU, a key player in the EU referendum, was unable to abide by these rules.
"Leave.EU exceeded its spending limit and failed to declare its funding and its spending correctly.”
Banks called the Electoral Commission’s findings "a politically motivated attack on Brexit and the 17.4m people who defied the establishment to vote for an independent Britain."
He added: "The EC went big game fishing and found a few 'aged' dead sardines on the beach. So much for the big conspiracy!
"What a shambles, we will see them in court."
Tory MP Damian Collins, chairman of the Digital, Culture, Media and Sport Select Committee, said Mr Banks and Andy Wigmore, Leave.EU's communications director, would face questions from the committee next month.