The billionaire who donated over $10 million to a campaign urging the U.K. to vote to leave the European Union is now the subject of a criminal investigation into the "true source" of the funds, raising questions of whether foreign money could have swayed the Brexit referendum.

On Thursday, Arron Banks was referred to Britain's National Crime Agency (NCA), the body that investigates serious and organized crime, by Britain's Electoral Commission. Banks is the biggest donor in British political history and a co-founder of Leave.EU, a campaign group for Brexit that was fronted by Nigel Farage during the 2016 referendum campaign. (There is no suggestion that Farage is under investigation or suspected of any wrongdoing.)

Banks said in a statement that he welcomes the investigation and is confident it will debunk the "ludicrous allegations" against him.