In Mandy: The Unauthorised Biography, Paul Routledge, a former political correspondent of The Independent on Sunday, will claim that last autumn Mr Mandelson "shifted gear" in his "long-term plan" to set his sights on becoming Prime Minister.
Mr Mandelson had wanted to oust Robin Cook as Foreign Secretary but, after joining the Cabinet, he raised his ambitions by preparing for a move to the Treasury, the book will claim.
Tony Blair was to move Mr Brown "reluctantly" to the Foreign Office before the general election, making room for Mr Mandelson to move from the Department of Trade and Industry to the Treasury, leaving him as the Prime Minister's heir apparent.
The claims were treated sceptically at Westminster last night and dismissed as "fanciful and absurd" by Mr Mandelson's friends, but theallegations underline the extent to which the Government is finding it difficult to refocus on policy.
Meanwhile it was reported last night that Mr Mandelson was pessimistic about the chances of an early comeback in the face of backbench opposition. The Parliamentary Labour Party is to meet the Prime Minister at Westminster today, and MPs are expected to urge Mr Blair not to bring back Mr Mandelson, at least until after the next election.
Mr Mandelson may have accepted that he will not be able to return to the Government as soon as he would have liked but friends say he is miserable because his only aim was public service.
He is also angry at suggestions that he is not apologetic enough about the loan and the damage it has inflicted.Reuse content