Martin Edwards, the former Manchester United chairman arrested as part of a money-laundering probe, wrote late last year in support of disgraced financier Michael Brown to the banking giant investigating him.
Mr Edwards was arrested earlier this year and released on police bail. He was subsequently released from bail, however, after the City of London police - who are leading the money-laundering investigation - informed him that they now believed him to be a victim of fraud, rather than a perpetrator. At the time, he told The Independent on Sunday he had been through "a pretty awful time".
Mr Edwards became embroiled in the probe through his involvement with Mr Brown, the former Liberal Democrat donor awaiting trial in Wormwood Scrubs.
HSBC started civil proceedings against Mr Brown and his company, 5th Avenue Partners, last autumn and the bank had his assets frozen. The case is understood to revolve around $45m (£25m) invested in Mr Brown's companies by Kevin So, Yan Lucy Lu - both believed to be based in China - a retired American lawyer, Robert William Mann, and Mr Edwards.
Court documents seen by The Independent on Sunday include a letter from Mr Edwards written in October and witnessed by his then solicitors Peters & Peters. It was sent in reply to an earlier letter from HSBC. The bank was concerned over whether Mr Brown was using the investors' money as specified.
Mr Edwards' response stated that he had known Mr Brown "for some considerable time" and that he was "content" the money had been used "in Mr Brown's discretion".
Mr Edwards, who appointed Sir Alex Ferguson and left the club in 2000, was unavailable for comment.
Mr Brown, who is Scottish but lives in Spain, left London before Christmas for his home in Majorca. HSBC, meanwhile, approached the Crown Prosecution Service last month to obtain a warrant for his arrest. Charges against Mr Brown include perverting the course of justice and forgery.
While the CPS assessed the application, two other alleged offences came to light: obtaining a passport by deception and making a false declaration to obtain a passport.
Mr Brown, 40, was extradited from Majorca last week and is scheduled to stand trial in October over the criminal charges.
The civil case will resume then as well.
Mr Brown's offices and home in Majorca were raided last week and a number of items were seized, including a Porsche Cayenne, a Bentley, a yacht, paintings and documents.