London: Leave.EU has been fined £70,000 (Dh348,745) and its chief executive has been referred to the Metropolitan police after the Electoral Commission found it had breached multiple counts of electoral law during the referendum to leave the European Union.

The investigation found that Leave.EU, which was cofounded by Arron Banks, unlawfully exceeded its statutory spending limit by at least 10 per cent and delivered incomplete and inaccurate spending and transaction returns.

The group campaign chief, Liz Bilney, one of Banks’s closest associates, faces a police investigation. The commission said it had reasonable grounds to suspect she “knowingly or recklessly signed a false declaration accompanying the Leave.EU referendum spending return”.

Banks responded by accusing the commission of a “politically motivated attack on Brexit” and threatened to take legal action.

Scotland Yard confirmed the commission had referred a potential criminal offence under section 123(4) of the Political Parties, Elections and Referendums Act 2000.

A spokeswoman said: “This matter will be subject to assessment by officers from the special inquiry team.”

The commission said the “unlawful overspend” was at least £77,380 but may well have been considerably higher.

It said services the group received from the US campaign strategists Goddard Gunster before the referendum officially began 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 inaccurately reported three loans it had received from companies controlled by Banks.

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.”

However, the commission found no evidence that Leave.EU received donations or paid-for services from Cambridge Analytica for its referendum campaigning and found that the relationship with the political campaign agency “did not develop beyond initial scoping work”.

The investigation instead concluded that Cambridge Analytica, which collapsed this month after a global scandal about its use of Facebook data, repeatedly attempted to obtain work from the Brexit campaign, but Banks never made good on his promises.

According to the report, Cambridge Analytica then became concerned Leave.EU was exaggerating their relationship “to bolster its own credibility” and asked the campaign to tone it down.

The Labour MP Liam Byrne has written to the commission asking why it failed to investigate the source of Banks’s financial contribution to Leave.EU.

He also called for a prosecution under joint enterprise rules to target those who enabled the offence. He said: “It seems highly unlikely that in this case the named responsible officer was acting on purely her own designs.”

Bob Posner, the 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 the fine would have been bigger but for a cap on the amount the commission can levy.

Posner said: “The rules we enforce were put in place by parliament to ensure transparency and public confidence in our democratic processes. 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. These are serious offences. The level of fine we have imposed has been constrained by the cap on the commission’s fines.”

Banks said: “The Electoral Commission is a ‘Blairite swamp creation’ packed full of establishment ‘remoaners’ that couldn’t quite make it to the House of Lords.