Mr Cook invited Mr Mandelson to take on the unpaid post when they met for dinner at the Foreign Secretary's London residence earlier this month. The dinner was arranged before Mr Mandelson resigned as secretary of state for trade and industry last month, in an attempt to build bridges between the Foreign Office and the DTI.
In the past, relations between the two men have sometimes been strained. But Mr Cook's allies reject a claim by his former wife, Margaret, that he "hated" Mr Mandelson, whose critics claim that before his downfall he wanted to take over as foreign secretary before the next general election.
Yesterday friends of both men dismissed claims in Margaret Cook's autobiography, A Slight and Delicate Creature, that Mr Mandelson intervened in her divorce negotiations with the Foreign Secretary. She suggested that Mr Mandelson helped her to win sole ownership of the family house and unwittingly encouraged her to write the book.
Mr Mandelson is anxious to play down his new role for Mr Cook, telling friends: "I will not be a roving ambassador or a proto-Foreign Secretary." He intends to give lectures at conferences throughout Europe - for which he can be paid now that he is no longer a minister - and will report back to Mr Cook on his contacts with other EU politicians.
A friend of the Foreign Secretary said last night: "He values talking to Peter because of his contacts in Europe, and they agreed to keep in touch. Peter is a valued colleague."
Mr Mandelson fears his new role will come under attack in the Eurosceptic press, because of his pro-European views and support for British membership of the single currency.
The MP for Hartlepool believes any backbencher could have taken on the role for Mr Cook, but his work will worry some Labour MPs who believe he is trying to rehabilitate himself too quickly.
Yesterday John McWilliam, MP for Blaydon, urged Tony Blair to declare that Mr Mandelson would not return to the Cabinet before the next general election.
Rosie Winterton, MP for Doncaster Central and a close ally of John Prescott, the Deputy Prime Minister, said of Mr Mandelson: "His problem is that the media is so obsessed by him that it may well be that they don't allow him to lead the life of an ordinary backbencher. That may well prevent him from ever making a comeback."
t Charlie Whelan, who resigned as Gordon Brown's press secretary despite denying that he leaked details of Mr Mandelson's pounds 373,000 personal loan from Geoffrey Robinson, suggested yesterday that he may return one day to work for the Chancellor.
Interviewed on BBC TV's Breakfast With Frost programme, Mr Whelan said he would not be advising the Chancellor "in the near future". Asked whether he might be called back to government one day, he replied: "Well, you never know."Reuse content