Benjamin Wegg-Prosser, former special adviser to Mr Mandelson, believed that his boss would be hounded out of office by the media even had he not breached the rules on ministerial conduct.
In a television programme tomorrow, Mr Wegg-Prosser says he knew there would be "a lot of pressure" on Mr Mandelson "because the newspapers would put a certain construction on it which would not look good for him and which would make it difficult for him to stay in government".
Mr Wegg-Prosser's father, a solicitor, had drawn up the loan agreement between Mr Mandelson and Mr Robinson, the former paymaster general.
Robert Harris, the author and a close friend of Mr Mandelson, tells the same programme that the former secretary of state for trade and industry was not focusing on the loan because he had been "outed" as a homosexual by Matthew Parris on television.
"He was, in the period before the [loan] revelation was made, worried about his private life," Mr Harris says. "He was focused on that. Actually, nobody cares about sexuality in politics any more. They do - oddly enough - care about money and he followed the wrong ball."
Politicians interviewed suggest Mr Mandelson will make a cabinet comeback. Lord Sawyer, the former Labour general secretary, says Mr Mandelson could "flower again" and be better off after two years "doing the nuts and bolts" of an MP's job.
The Real Peter Mandelson will be shown on Channel 4 at 8pm tomorrow. It is presented by Donald Macintyre, whose biography, Mandelson, is published by HarperCollins.Reuse content