Mr Mandelson could have been legally obliged to mention the loan from Mr Robinson when applying for the mortgage. Speaking shortly after resigning, the former minister said he failed to recall what he had said on the form, but had never intended to mislead anyone.

Since then it has been alleged Mr Mandelson did not sell his former home in Clerkenwell until three months after he bought the £475,000 Notting Hill mansion. That would mean he was paying two mortgages despite earning only about £40,000 at that time.

Mr Mandelson has since written to the building society to clarify what he wrote on his application form, but has made no further public comment to clarify the matter.

Tories are planning to draft a complaint to the Parliamentary Commissioner for Standards over five free flights Mr Mandelson took in a private jet.

Lingerie millionaire Linda Wachner, who is behind the former minister's free flights, describes Mr Mandelson as a friend.

His office says the flights should not have been declared as they had nothing to do with his role as an MP or minister.

Newspapers from Rupert Murdoch's News International stable have claimed Mr Robinson offered home loans to three other Cabinet members.

The Sunday Times quoted an anonymous minister as saying he had been offered a cheap loan, which would implicitly be repaid by loyalty to Chancellor Gordon Brown.

So far, no firm evidence of this has been offered, nor has any publication sought to name the ministers allegedly approached by the former paymaster general.

Conservatives are also asking whether the millionaire MP also funded Gordon Brown's team in opposition.

Within hours of Mr Mandelson resigning his successor had been announced as the Blairite Stephen Byers.

However, Mr Robinson's replacement has yet to be announced.

Mr Robinson finally resigned after months of opposition MPs calling for him to quit over his business affairs, which would have given the prime minister plenty of time to consider a successor.

Two reasons are offered for the delay. First, the man widely tipped to replace Mr Robinson, Geoff Hoon, is a Blairite and could be opposed by Chancellor Gordon Brown and others at the Treasury.

Secondly, Mr Robinson drew no salary for his government work. A successor would have to be paid and the budget for ministerial salaries is believed to already be stretched.

Another key question that remains unsatisfactorily resolved.

The secret loan from Mr Robinson to Mr Mandelson two years ago features in the first chapter of Mirror journalist Paul Routledge's hostile biography of the former trade secretary.

The Guardian, who broke the story, claim they obtained details of the loan from an independent source.

Some have suggested sources close to Mr Mandelson may have leaked the story, although given the consequences that is unlikely.

The main culprit according to the press is Charlie Whelan, Gordon Brown's press secretary, although he has denied anything to do with the leak.