Brexit campaign group Leave.EU has been fined £70,000 for breaking rules on spending during the EU referendum campaign, the Electoral Commission has announced.

The group, co-founded by former Ukip backer Arron Banks, overspent by at least £77,380 – 10% over the limit for non-party registered groups – an investigation found.

Banks branded the investigation a “joke” and said it had “fired an arrow into the wall and painted a target around the arrow”.

The probe concluded that the pro-Brexit campaign incorrectly reported what it spent during the referendum period, unlawfully exceeded its statutory spending limit by at least 10% and delivered incomplete and inaccurate spending and transaction returns.

The commission said the “unlawful over-spend” was at least £77,380 but “may well have been considerably higher than that”.

It said services the group 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 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. ”

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

Banks told BBC Radio 4′s Today programme: “The entire commission is composed of former MPs, liberal MPs, the SNP, former Labour leaders of councils, all sorts of people that all believe in Remain.

“I can tell you one thing: we will not be letting matters rest here, we will be going to court to challenge this. It’s certainly not the last word on the subject.”

Damian Collins, chair of the Commons Digital, Culture, Media and Sport committee, said it would question question the Electoral Commission on Leave.EU’s activities next week, with Banks and campaign chief Andy Wigmore appearing next month.

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.

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.”

Leave.EU was one of the biggest pro-Brexit groups although Vote Leave was the official campaign organisation.

Responding to the ruling, 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, but managed to get their noses in the trough via appointment to public bodies like the Electoral Commission.”

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.”

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