A pro-Brexit activist and donor had an investigation into him by the National Crime Agency (NCA).
The agency found that there was no evidence that any criminal offenses were committed by Arron Banks.
Britain's election regulator, the Electoral Commission, referred the investigation to the NCA last November.
He argued that there were "reasonable reasons" to suspect that Banks was not the true source of £ 8 million in loans to Better for the Country, which ran his group Leave.EU during the 2016 referendum campaign.
"A number of criminal offenses may have been committed," he added, also referring to Leave.EU chief executive Elizabeth Bilney for investigation.
Bob Posner, the agency's director of political finance, said he suspected that up to £ 2.9 million came from "disallowed sources" and that Banks and Bilney "consciously concealed the true circumstances under which this money was provided."
But after the NCA interviewed the two, Banks did not break the law because it had a "legal right" to order transactions.
He confirmed that he will not take "any further action" against Banks or Bilney, or the organizations involved.
In a statement, Mr. Banks said: "For almost a year, I was subjected to an endless media campaign, suggesting that I behaved inappropriately during the 2016 referendum campaign.
"All this because the Electoral Commission, having found no impropriety, forwarded my file to the NCA.
"They made false and outrageous allegations that I was not the real source of funds during the campaign and that" several crimes may have been committed. "
"No wrongdoing of any kind has been found and I intend to act against those who behaved so outrageously."
An NCA statement said: "The NCA has received no evidence to suggest that Mr. Banks and his companies have received third party financing to finance the loans, or that he acted as an agent on behalf of third parties.
"There have also been media reports alleging that Banks was involved in other foreign business-related criminality. The NCA neither confirms nor denies that it is investigating these reports."
The Electoral Commission responded by saying that it was "concerned about the apparent weakness of the law, highlighted by this result of the investigation, which allows foreign funds for British politics."
A spokesman added: "We made recommendations that would restrict campaign funding rules and prevent violations.
"We urge UK governments to act on these recommendations to support voter confidence."
Earlier this month, the Metropolitan Police found that the Leave.EU campaign committed technical violations of the electoral law about their return to EU spending, but would not face further action.
The Election Commission led the campaign to police its spending during the 2016 referendum and fined £ 70,000.