BBC Political correspondent John Pienaar: The betting is that he will live to spin another day

The Mirror published the memo - written four days after former Labour leader John Smith died in May 1994 - as part of the first excerpt from Paul Routledge's unauthorised biography of Mr Mandelson.

It is a further embarrassment to Mr Mandelson, who resigned just before Christmas after admitting accepting a £373,000 loan from former paymaster general Geoffrey Robinson, who also quit.

Peter Mandeslon quit over loan

The Mirror's editor Piers Morgan says Mr Mandelson - a close friend of Prime Minister Tony Blair - acted as "kingmaker" following Mr Smith's death.

'Knifed in the back'

But he says the Hartlepool MP was a friend of Mr Brown's at the time and says the letter shows he "knifed him squarely between the shoulder blades".

In the letter, Mr Mandelson flattered Mr Brown, calling him "the biggest intellectual force and strategic thinker the party has".

Mirror editor Piers Morgan: "Paul Routledge's book has brought things to a head in the Labour party"

But he went on to say Mr Brown did not have the "southern appeal" Mr Blair had and which party strategists felt was vital if Labour was going to win crucial marginal seats in southern England.

Mr Mandelson warned the shadow chancellor if he stood it would split the vote and allow in people such as Foreign Secretary Robin Cook.

He said Mr Brown would be "blamed by the media" for creating the split.

Hunt on for 'sneak'

The leaked letter is bound to reopen divisions between the Brown and Blair camps within Labour.

Labour's spin doctors will no doubt be desperate to find out who leaked it to Mr Routledge.

Mirror editor Piers Morgan: "Peter Mandelson clearly played the role of kingmaker when John Smith died"

Mr Morgan says he does not know who the mole is and has not even asked Mr Routledge.

Speaking on BBC Radio 4's Today programme, Mr Routledge said the leak had not come from Mr Whelan, who had pleaded with him not to write the book as he would be blamed for the content.

Charlie Whelan is made favourite for the leak in the Mirror

Mr Routledge said someone had stolen the story of the secret loan from proofs of his book, which was delivered to the wrong office in the Commons.

He said: "I was waiting for something to happen, because I knew somebody had got hold of it.

"Then, I knew Mandelson's people were offering a 'big story' about Robinson which they wanted to do before Christmas."

Pre-emptive strike

He added: "Whoever stole the story talked to the Mandelson camp and they decided rather than waiting to come out in the book they would take a pre-emptive strike."

Paul Routledge tells BBC Radio 4's Today programme: "Whelan begged me not to write the book"

A spokesman for Mr Mandelson said on Monday: "There is no mystery about this memo which has been in the public domain for over a year. Peter wrote it, Gordon received it and it was unwittingly passed on amongst a pile of other papers to Mr Routledge.

"Peter and Gordon won't let the Mirror play one off against the other to hurt the government. Everyone involved in the issue is relaxed by the re-publication of this document."