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 the necessary 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".