The National Crime Agency (NCA) has launched an investigation into pro-Brexit donor Arron Banks and his Leave.EU campaign for "suspected electoral law offences".
"Better for the Country", the organisation that ran Leave.EU, and its chief executive Elizabeth Bilney were among a group of people and companies referred by the Electoral Commission on Thursday.
The NCA said it would not normally investigate such matters, but that the "nature" of inquiries and "potential for offences to have been committed other than under electoral law " warranted its intervention.
The Electoral Commission had been examining a reported £2m loan from Arron Banks and his group of insurance companies to Better for the Country.
It was also looking at a reported £6m loaned to the organisation by Mr Banks alone.
Nearly half of that – £2.9m – was used to fund spending in the 2016 EU referendum on behalf of Leave.EU and donations to other campaign groups, the commission said.
It claimed there were "reasonable grounds to suspect" that Mr Banks was not the true source of either loan and that "a number of criminal offences may have been committed".
Bob Posner, the Commission's director of political finance, said he suspected some of the money came from "impermissible sources" and that Mr Banks and Ms Bilney "knowingly concealed the true circumstances under which this money was provided".
Mr Banks welcomed the NCA investigation, saying it was an opportunity to clear up the matter.
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He promised he had not received any foreign money and that he is a UK taxpayer.