Two scorpions are back in same bottle

But today's Cabinet comeback proves the pair's relationship has always been much more complex than the image of "betrayed" and "betrayer".

The EU Trade Commissioner's return also underlines the resilience and political appetite of a man who once declared he was a "fighter, not a quitter".

Mr Mandelson and Mr Brown worked closely during the Eighties and early Nineties, with the former spin chief believing the young Scot was a future Labour leader. It was John Smith's sudden death in 1994 that triggered their feud, as Mr Mandelson switched allegiances to Mr Blair.

Mr Mandelson found himself caught between loyalty to Mr Brown and the recognition Mr Blair would be a more popular choice as leader. Emotionally torn, he wrote a long letter to Mr Brown in which he promised to back the then shadow chancellor's claim to the succession - but, in a killer phrase, suggested there would be "presentational difficulties" in having a dour Scot not a media-friendly Englishman as premier.

Ever since he sided with Mr Blair, Mr Mandelson has been viewed with contempt by Brown allies. At the height of the feud the pair were described by one colleague as "like scorpions in a bottle - only one will get out alive".

Others say their relationship has been more complicated. When he first quit the Cabinet Mr Mandelson was in tears as he phoned Mr Brown to ask for advice on his resignation letter. The pair have shared a mutual respect that has increased in recent months.

The new Business Secretary's longtime friend, novelist Robert Harris, has pointed out that Mr Mandelson was a victim rather than a player in the Brown-Blair battle: "I don't think one has to be a brilliant psychoanalyst to see that there was quite a lot of transfer of [Brown's] anger on to Peter that couldn't be inflicted on Blair himself."

With Mr Blair out of the way, relations are easier. Only last week, Mr Mandelson said: "It would be odd if we didn't discuss domestic issues. We have a shared interest in Labour's fortunes."

He was at the Labour conference in Manchester last month but pulled out of a rally of the Blairite Progress group, telling friends he did not want to be seen to undermine the Prime Minister.

Today's promotion is yet another return from the wilderness. Once dubbed "The Prince of Darkness" by Brownites, his new peerage guarantees he will be around in frontline politics for many years.

One of the architects of New Labour, he was made a minister in 1997 and given responsibility for the Millennium Dome. He became trade secretary but was forced to quit in 1998 after it emerged he had a secret loan of £373,000 from ministerial colleague Geoffrey Robinson.

Mr Blair brought him back in 1999 as Northern Ireland Secretary, but he had to resign in 2001 over allegations of misconduct over a passport application for Indian tycoons the Hinduja brothers.

He staged a further comeback when he was appointed EU Trade Commissioner by Mr Blair.