- Copyright: PA Media
Meanwhile the Twitter account of Arron Banks, the founder of the pro-Brexit campaign Leave.EU, has been suspended after being hacked.
In a statement posted by the campaign's official account, Mr Banks said "personal data" had since been shared via Twitter.
He also accused the social network of failing to tackle the issue.
Mr Banks said Twitter had "deliberately chosen" to leave his personal information online.
- Copyright: PA
The company which the Electoral Commission suspects may have been behind Britain's biggest political donation has no registered address - meaning it has failed to fulfil a key legal requirement - the BBC has found.
Rock Holdings is a firm registered in the Isle of Man and controlled by Leave.EU founder, Bristol-based Arron Banks.
The Electoral Commission's ongoing investigation of Leave.EU funds was referred to the National Crime Agency.
Mr Banks denies any wrongdoing.
By John Sweeney
Leave.EU co-founder Arron Banks tells the BBC all the money he donated to it was UK-generated.
Leave.EU chief executive Liz Bilney is "confident" she and campaign founder Arron Banks - who is also under investigation over Brexit vote spending - will be exonerated.
The National Crime Agency is investigating Bristol-based businessman Arron Banks and his Leave.EU campaign for alleged offences committed at the 2016 EU referendum.
The Leave campaigner and another senior figure in the campaign, Liz Bilney, were referred the agency by the Electoral Commission.
Mr Banks has consistently denied any wrongdoing in connection to the referendum campaign.
Leave.EU was a separate campaign to the official Vote Leave organisation. Their headquarters are based near Cribbs Causeway.Copyright: BBC