Age: 43.
Who is he? British Minister without Portfolio and the PR genius behind Labour's general election victory. He's the grandson of Herbert Morrison, fiery Labour MP who served in Winston Churchill's wartime cabinet.
Why is he blinking in the spot- light?: Admitted he had been left "minding the shop" with Deputy Prime Minister John Prescott while Tony Blair was taking a holiday.
So? When asked why a non-Cabinet minister was becoming the "face of the government" he turned on BBC reporters. Resulted in some 30 complaints about his "arrogance and pompous manner". Shadow culture secretary Francis Maude accused him of "midsummer madness verging on megalomania".
Peter's style: A confidant of the Prime Minister, he helped Blair win party leadership after the death of John Smith in 1994. A worker behind the scenes, he has probably more political instinct than anyone else in the Labour Party. Dubbed the government's trouble-shooter, he's sent to sort things out at the first sign of trouble. Was handed control of the Millennium project in Greenwich when Blair insisted it should be saved.
Also known as? "The Prince of Darkness"; "Rasputin"; "Bobby", after the brother of former US president John F. Kennedy
Questions: Before the election, shadow Cabinet ministers were openly at odds with his role. Clare Short is believed to have been talking about him when she referred to "dark forces" surrounding Blair.
But: Mandelson appears to revel in the hatred he attracts. Labour left-wingers see him as the destroyer of socialist values. Otherwise fearsome political correspondents at Westminster say he tries to bully them.
Peter's ambitions: Widely regarded as wanting Chancellor Gordon Brown's seat on the National Executive Council at the Labour party conference this autumn. Seen and criticised as an unaccountable "prime minister in waiting".
Last word to Peter: "I feel in a sense as if I've been picked out of the middle-ranks of the army and put on some particularly testing and challenging assault course . . . just to see whether I survive and can get through it all."