Campaign group Leave.EU has been fined a maximum £70,000 for breaching electoral law during the Brexit referendum campaign.
And its chief executive Liz Bilney has also been referred to police.
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 had also delivered incomplete and inaccurate spending and transaction returns, said the commission.
Leave.EU had failed to include at least £77,380 in its spending return, said the commission, which also speculated that the overspending may have been much higher.
It added that it suspected Bilney had ‘knowingly or recklessly signed a false declaration accompanying the Leave.EU referendum spending return’.
Leave.EU was not the official Brexit campiagn group during the referendum.
It was initially endorsed by former UKIP leader Nigel Farage and was founded by millionaire businessmen Arron Banks.
In response, Mr Banks accused the commission of a ‘politically motivated attack on Brexit’ and threatened to take legal action in return.
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However, the commission found no evidence that Leave.EU had received any donations or worked with Cambridge Analytica, which this month announced it was closing after the fallout from allegations that personal data from millions of people had been harvested.
Leave.EU’s fine is the record-equalling largest handed out and matches the amount the Conservative Party paid last year for election offences.
The June 2016 vote saw Britain vote to leave the EU. Almost 52% voted to split with Brussels with 48% choosing to stay.