The commission said it suspected criminal offences may have been committed, and the responsible person, Leave.EU CEO Liz Bilney, had been referred to the police.
The group also incorrectly reported what it spent during the campaign and inaccurately reported three loans it had received, according to the commission.
This means Leave.EU exceeded its spending limit by at least 10%, although the Electoral Commission believes the unlawful overspend may have been considerably higher.
One of the founders Leave.EU arrone banks called the penalty of the election Committee "politically motivated persecution". The dates the transactions were entered into, the repayment date, the interest rate and the provider of the transactions were all incorrectly reported.
But it found no evidence that controversial tech firm Cambridge Analytica had made donations or provided paid-for services, and its role was limited to "initial scoping work".
Leave.EU said the total alleged overspend represented less than 0.1 per cent of the total campaign finance spend and was a risible attempt to portray the campaign as breaking the rules.
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"Leave.EU exceeded its spending limit and failed to declare its funding and its spending correctly", Posner said. These are serious offences. The Electoral Commission has fined the campaign organisation a total of £70,000 for failures in its declaration, with the regulator noting the level of penalties "has been constrained by the cap on the commission's fines".
Under the Political Parties, Elections and Referendums Act 2000 (PPERA) it was the responsibility of all campaigners to ensure that an accurate and complete campaign spending return was submitted to the Electoral Commission by the statutory deadline following the European Union referendum.
The investigation into Leave.EU began in April 2017. Leave.EU reported spending of 693,094 pounds, 6,906 pounds under the spending limit.
However, the commission pledged to keep this aspect of the investigation under review in case further evidence about Cambridge Analytica comes to light.
Campaign bags are placed on seats before the start of a Leave.EU campaign news conference in central London, Britain November 18, 2015.
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.
- Leave.EU paid for services from the USA campaign strategy firm Goddard Gunster that should have been reported in its spending return but were not.