Leave campaigner Arron Banks is being investigated by the National Crime Agency for "suspected criminal offences" committed during the Brexit referendum.
The probe has been launched after the elections watchdog said it had reasonable grounds to suspect that Mr Banks was not the true source of £8 million worth of loans made during the campaign.
Leave.EU, co-founded by Mr Banks, its chief executive Elizabeth Bilney and the organisation that ran it, Better for the Country, are also being investigated by the NCA after a review carried out by the Electoral Commission.
Bob Posner, the Commission's director of political finance, 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."
In a statement, the NCA said it had "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."
Mr 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".