The head of the group has also been referred to police
The Leave.EU campaign in the UK has been fined stg£70,000 (€79,514) for breaches of electoral law during the EU referendum campaign, with the head of the pro-Brexit group referred to police.
The British Electoral Commission announced the findings of its investigation on Friday, with Leave.EU found to have incorrectly reported what it spent at the EU referendum.
It exceeded its statutory spending limit and delivered incomplete and inaccurate spending and transaction returns.
The group, which was not the official Brexit campaign, failed to include at least stg£77,380 (€87,821) in its spending return.
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.
The investigation also found the group, which was initially endorsed by former UKIP leader Nigel Farage and was founded by businessmen Arron Banks and Richard Tice, inaccurately reported three loans it received.
In addition, Leave.EU failed to provide the required invoice or receipt for 97 payments of more than stg£200 (€227), totalling stg£80,224 (€91,055).
During the course of its investigation, the commission said it found "reasonable grounds to suspect" Leave.EU's chief executive Liz Bilney committed criminal offences.
She has been referred to the Metropolitan Police.
Scotland Yard confirmed the commission has referred a potential criminal offence.
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.
Bob Posner, the Electoral Commission's director of political finance and regulation, 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."