Banks, a millionaire who made his fortune in the insurance industry, bankrolled the Leave.EU campaign and its parent organisation, Better for the Country.
The elections watchdog, which had been investigating the funding of campaigns during the 2016 referendum, said on Thursday that it had found evidence of multiple suspected offences.
“The investigation focused on £2m reported to have been loaned to Better for the Country by Arron Banks and his group of insurance companies and a further £6m reported to have been given to the organisation, on behalf of Leave.EU, by Arron Banks alone,” the Electoral Commission said in a statement.
“£2.9m of this money was used to fund referendum spending on behalf of Leave.EU and donations to other campaign groups during the EU referendum.”
It said that its investigations had concluded that Banks “was not the true source of the £8m loans made to Better for the Country” as part of a possible criminal conspiracy.
Banks, 52, was named by the Commission alongside Elizabeth Bilney, Leave.EU’s chief executive, “and others involved in Better for the Country”.
Bob Posner, the Electoral Commission’s director of political finance and regulation and legal counsel, said: “We have reasonable grounds to suspect money given to Better for the Country came from impermissible sources and that Mr Banks and Ms Bilney, the responsible person for Leave.EU, knowingly concealed the true circumstances under which this money was provided.
“This is significant because at least £2.9m of this money was used to fund referendum spending and donations during the regulated period of the EU referendum.
“Our investigation has unveiled evidence that suggests criminal offences have been committed which fall beyond the remit of the Commission. This is why we have handed our evidence to the NCA to allow them to investigate and take any appropriate law enforcement action. This is now a criminal investigation.
“The financial transactions we have investigated include companies incorporated in Gibraltar and the Isle of Man.
“These jurisdictions are beyond the reach of the Electoral Commission for the purpose of obtaining information for use in criminal investigations or proceedings.”
The findings prompted the Commission to refer Banks and Bilney, together with other individuals and the organisations involved, to the National Crime Agency.
The National Crime Agency said in a statement: “The NCA has initiated an investigation concerning the entities Better for the Country (BFTC) and Leave.EU; as well as Arron Banks, Elizabeth Bilney and other individuals.
“This follows our acceptance of a referral of material from the Electoral Commission.
“Our investigation relates to suspected electoral law offences covered by that referral, as well as any associated offences.
“While electoral law offences would not routinely fall within the NCA’s remit, the nature of the necessary inquiries and the potential for offences to have been committed other than under electoral law lead us to consider an NCA investigation appropriate in this instance.
“This is now a live investigation, and we are unable to discuss any operational detail.”
Banks said he is “confident that a full and frank investigation will finally put an end to the ludicrous allegations levelled against me and my colleagues”.
Bilney accused the Commission of having a “biased approach” in its investigations and said she was confident she would be “exonerated”.
While Andy Wigmore, a spokesperson for Leave.EU, told the Guardian newspaper: “It’s completely to be expected. It will finally bring a head to all these crazy allegations made about us. We’re not worried.”