The MP for Hartlepool, his party's best-known spin doctor, has privately warned the deputy leader, John Prescott, that he will not agree with further reforms that Mr Mandelson is urging in a book published next month.
In the handwritten letter headed "In Confidence" Mr Mandelson submitted proofs of his book to the deputy leader because there is a passage about him in the second chapter.
He warns: "I know you will not agree with the way everything is put, but at least no one can accuse me of not coming clean with what I really believe!"
Mr Mandelson, whose brief includes presentation of policies, is to urge further moves towards the centre ground of politics by Labour in education, welfare, taxation and other areas and argue for a closer relationship with the Liberal Democrats.
Advance reports have already prompted fierce condemnation from Labour's left wing.
However, Mr Mandelson tells Mr Prescott: "Someone who's seen it [the proofs] told me they are surprised by how left-wing it is." Party insiders regard this as unlikely, nor do they take seriously his view that Mr Prescott would warm to the co-author, ex-SDP councillor Roger Liddle.
Mr Mandelson insists: "He is someone, by the way, who you would like if you knew him." Mr Liddle quit the Labour Party for the SDP over hard- left policies in the Eighties, returning when modernisers came to power.
The word at Westminster is that Mr Mandelson was told by Tony Blair to tone down the book. "I suspect that it will be a much less exciting book than would have been the case," said Ken Livingstone MP. "But I am sure that remains his long-term strategy: the creation of a two-party system, with Labour being no more radical than the American Democrats.
"That is what he really believes in, and given that he is the closest person to Tony Blair in the party, you have to worry about how much the leader might agree with it.
"Mandelson's strategy is not workable. The party and the trade unions will not have it."Reuse content