However, the Electoral Commission probe found no evidence Leave.EU received donations or paid-for services from Cambridge Analytica for its referendum campaigning and the group's relationship with the firm did not develop beyond initial scoping work.
Mr Banks claimed that the make up of the Electoral Commision made it biased against Brexit, stating it was a "collection of former politicians from the Liberal party, the SNP, ex-Labour leader councils".
The findings come after the Electoral Commission completed its investigation into Leave.EU and it had exceeded its statutory spending limit during the 2016 referendum by at least 10%.
It added that it suspected Bilney had "knowingly or recklessly signed a false declaration accompanying the Leave.EU referendum spending return".
The campaign, associated with Nigel Farage and businessman Arron Banks, was allowed to spend Â£700,000 during the campaign, but exceeded this figure by at least Â£77,380, the investigation found.
It revealed the organisation also failed to provide invoices or receipts for a further 97 payments it received of more than £200, which together totalled £80,244.
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The Electoral Commission said the penalties imposed on Leave.EU had been constrained by the cap on the watchdog's fines.
"It is therefore disappointing that Leave.EU, a key player in the EU referendum, was unable to abide by these rules". These are serious offences.
Leave.EU's fine is the record-equalling largest handed out and matches the amount the Conservative Party paid previous year for election offences.
"We view the Electoral Commission announcement as a politically motivated attack on Brexit and the 17.4 million people who defied the establishment to vote for an independent Britain", he said.
The Commission found Leave.EU inaccurately reported three loans it had received. Services the group received from the U.S. campaign strategy firm Goddard Gunster were not included in the spending return, despite a proportion of them having been used during Leave.EU's referendum campaign.
FILE PHOTO: Brittany Kaiser of Cambridge Analytica, Brexit campaigner Aaron Banks, Gerry Gunster, a Washington-based strategist hired by the Leave.EU campaign, and Liz Bilney, CE of Eldon Insurance Services during a Leave.EU news conference in central London, Britain, November 18, 2015.
"I can tell you one thing: we will not be letting matters rest here, we will be going to court to challenge this".