Mr Mandelson resigned just before Christmas after admitting it was a mistake to have accepted a £373,000 loan from millionaire Labour MP Geoffrey Robinson.

Mr Robinson, who also quit as Paymaster General, is being investigated by the Department of Trade and Industry.

The Mirror and The Guardian are at loggerheads over the Mandelson story

The loan was to enable Mr Mandelson to buy a house in London's fashionable Notting Hill Gate. But it remains unclear exactly how details of Mr Mandelson's financial dealings reached the public domain.

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The dispute between The Mirror and The Guardian, two of Labour's most stalwart supporters in Fleet Street, centres on an unofficial biography of Mr Mandelson by political journalist and Mirror columnist Paul Routledge.

The Mirror, which is to serialise the book starting on Monday, said on Saturday a parcel containing the 300-page draught manuscript had been broken into in Mr Routledge's office at the House of Commons.

Mr Mandelson borrowed money from Mr Robinson to buy this Notting Hill Gate flat

It said the thief passed details of the book - including the loan revelations - to Mr Mandelson, who leaked the story himself in order to limit the damage. But he was forced to resign.

Mr Routledge told The Mirror he had ironically decided to leave out details of Mr Mandelson's loan from Mr Robinson in his finished book.

'Suspicion on Guardian staff'

He said the person who leaked the details from his book was "almost certainly" a journalist sympathetic to Mr Mandelson.

Mr Routledge writes: "Suspicion at Westminster is focusing on staff of the Guardian group."

But Guardian editor Alan Rusbridger denies Mr Mandelson was the source of its original story on the affair.

He said: "The Mirror's claim that the Guardian stole this story from Paul Routledge's book is fantasy and is untrue.

"Paul Routledge cannot produce a shred of evidence to substantiate this ridiculous claim".