Partners at Arthur Andersen said there was "substance" behind the claim that the document was faked. Senior sources at the firm, who are receivers to Landhurst Leasing, a company at the centre of the investigation, said they had been unable to locate the loan document used in the programme.
The firm had also been unable to find any correspondence or other supplementary papers relating to the alleged loan. A partner said Landhurst's loan documents were normally laid out in a tabular format - unlike the document shown in the programme .
Andersen's backing casts a further shadow over Panorama and its reporter, Martin Bashir. Earlier this week, the BBC admitted that Mr Bashir had commissioned fake bank statements to be drawn up by a graphic designer shortly before his exclusive interview with the Princess of Wales last November. Mr Bashir was the reporter on Panorama's two "exposes" of Mr Venables' financial dealings in 1993 and 1994.
One of the entries in the fake bank statements showed a payment from a firm with the same name as one belonging to Eddie Ashby, Mr Venables' long-time business associate, to Alan Waller, a former head of security for Earl Spencer, the Princess's brother.
On his first programme in 1993 about the football coach, Mr Bashir alleged that Mr Venables had "decided to cheat" one of his former companies in order to obtain a pounds 1m loan from Landhurst. The document reproduced on screen purported to show a schedule of assets used as collateral for the loan and was accompanied by Mr Venables' signature. Mr Venables does not deny receiving a loan from Landhurst. He does maintain, though, that he did not do a "sale and leaseback" deal, as the programme claimed, involving assets he did not own. He is suing the BBC for libel over the allegation.
The BBC said last night: "We are confident that the information is authentic and we are not prepared to comment further because of the legal position."