The National Crime Agency said it has found no evidence of criminality after investigating a series of claims against Brexit campaign group Leave.EU and the businessman Arron Banks.
But it has not ruled out further investigations over allegations of involvement in “criminality related to business dealings overseas”.
The NCA said that no evidence of crime had been found concerning £8 million of EU referendum campaign funding after interviewing Banks and the Leave.EU campaign’s chief executive Liz Bilney.
It said that no further action would be taken against campaign groups Better for the Country Ltd or Leave.EU.
A referral was made by the Electoral Commission in October last year claiming that Banks was not the true source of the £8 million loans, and that the company through which they were made, the Isle of Man registered Rock Holdings Ltd, should not have been used to make donations because it is registered outside the UK.
The NCA said in a statement: “In carrying out its complex investigation the NCA analysed information, including banking information, which had not formed part of the Electoral Commission’s investigation. It also interviewed Mr Banks and Ms Bilney.
“In reaching its decision, the NCA obtained advice from the Crown Prosecution Service and expert independent counsel due to the technical nature of Political Parties, Elections and Referendums Act (PPERA) offences and the company structures involved.
“The NCA has found no evidence that any criminal offences have been committed under PPERA or company law by any of the individuals or organisations referred to it by the Electoral Commission.
“It will therefore take no further action against Mr Banks, Ms Bilney, Better for the Country Ltd or Leave.EU in respect of this specific matter.”
The inquiry concluded that Banks took a loan from Rock Holdings Ltd, which he owned, and was legally entitled to do so.
He was also allowed as an individual to then give the money to Better for the Country Ltd, which administered the Leave.EU campaign.
The NCA statement went on: “The NCA has not received any evidence to suggest that Mr Banks and his companies received funding from any third party to fund the loans, or that he acted as an agent on behalf of a third party.
But it added: “There have also been media reports alleging that Mr Banks has been involved in other criminality related to business dealings overseas. The NCA neither confirms nor denies that it is investigating these reports.”
Earlier this month the Metropolitan Police found that the Leave.EU campaign committed technical breaches of electoral law over its EU referendum spending return, but would face no further action.
The Electoral Commission referred the campaign to police over its spending during the 2016 poll and fined it a record-equalling £70,000.