The deal was done in 1996 - before Labour came to power last year - to enable Mr Mandelson to buy a London home.

Geoffrey Robinson: Under investigation

Mr Mandelson is now Trade and Industry Secretary and his department is investigating Mr Robinson's business affairs.

Shadow Trade and Industry secretary John Redwood asked: "How can Mr Mandelson be Britain's top company regulator when he has this kind of relationship with people undergoing scrutiny by his own department?"

John Redwood on the Today programme

Later, Mr Redwood told BBC Radio 4's Today programme: "I think we need a lot more information.

"We need to know from Mr Mandelson what the terms of the loan were.

"This is a huge amount of money, way outside the sum of money you would normally lend to an MP on a parliamentary salary and we need to know what strings were attached.

"We need to know whether Mr Robinson has bankrolled other senior members of government either when they were in opposition or now they are in government."

Malcolm Bruce on the Today programme

Liberal Democrat Treasury spokesman Malcolm Bruce described the disclosures as "disturbing".

He said: "Mr Mandelson should clearly have flagged up the potential conflict of interest as soon as his officials had notified him of the investigation into Mr Robinson's affairs."

He told Today Mr Robinson's position as paymaster general was now "untenable".

Former Conservative junior trade minister Neil Hamilton called on Mr Mandelson to prove he had no influence over the investigation into Mr Robinson's business dealings.

Peter Mandelson: No connection with examination of Geoffrey Robinson's affairs

Mr Mandelson insists he acted correctly, saying it had not led to a conflict of interest as he deliberately had nothing to do with the investigation.

He removed himself from any potential conflict of interest by telling the DTI's top official, Permanent Secretary Michael Scholar, he wanted "absolutely nothing to do with the DTI examination of Geoffrey Robinson's affairs" from the outset.

But Mr Hamilton, who resigned in October 1994 over the cash for questions row involving Mohamed al-Fayed, said Mr Mandelson should prove he is above suspicion by publishing evidence to back up his claims.

Neil Hamilton: There should be departmental minutes

He said: "We need to know when Mr Mandelson ruled himself out of consideration into Mr Robinson's affairs and when he did that - there should be departmental minutes to that effect.

"If there is not there should be an immediate investigation.

"There would be a note on the file if he had made a decision not to make any decisions relating to Geoffrey Robinson.

Tony Blair: Backing for Mr Mandelson

"If the note isn't published then we will draw our own conclusions that Peter Mandelson's assurances aren't worth the paper they are written on."

Mr Mandelson immediately won backing from Prime Minister Tony Blair who issued a statement saying his staff had carried out an investigation that cleared the trade secretary of any wrongdoing.

Peter Mandelson: Insists he ruled himself out of investigation

Scottish National Party Chief Executive Michael Russell said the instance of the loan was bound to raise concerns

Mr Mandelson, who was formerly in charge of government presentation, has admitted it would have looked better if he had declared the loan sooner.

He did not tell Mr Blair or Mr Scholar, until last Thursday.