First among unequals: Meet the most powerful person in Britain

Last Thursday evening, at a garden party at the grand Holland Park home of the PR chief Roland Rudd, Peter Mandelson was, in the words of one guest, the "cynosure of all eyes".

The party was packed with senior figures from the media, cabinet ministers and business leaders. But it was Lord Mandelson, the new First Secretary of State, and deputy prime minister in all but name, whose words they all craned to hear.

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Lord Mandelson's power now appears absolute: many believe he is, in fact, not the deputy but the real prime minister, having rescued Gordon Brown from a coup by persuading several cabinet colleagues not to jump ship. He has accrued a Whitehall powerbase so vast it includes more ministers – 11, including himself – than in the three departments of Transport, International Development and Energy and Climate Change put together.

Hours before the lavish Kensington party, the Business Secretary had, along with other ministers and hundreds of MPs and activists, attended Labour's gala evening at Stamford Bridge. Before sitting down to dinner, David Miliband, the Foreign Secretary, who can only wait for Labour to lose the next election, threw off his jacket to take shots at the Chelsea goal, attracting praise for his nimble footwork. Other ministers wandered around, attempting to put a brave face on the turbulence of the previous week.

But Mr Brown, during his speech to activists at the fundraising event, mentioned just one minister by name – Lord Mandelson. The PM, who knows he was saved by the Business Secretary's powers of persuasion, cracked jokes to give the impression that they are now the best of friends. Yet the mood of the evening was uneasy, and not just because of Labour's electoral difficulties. The man who a year ago Mr Brown hoped would reinvigorate his Cabinet can now make or break him. The peer's empire – or "Raj" as it was dubbed last week – is not just about the impressive line-up of ministers, or his 36-word title: Rt Hon Lord Mandelson of Foy in the County of Herefordshire and Hartlepool in the County of Durham, First Secretary of State, Secretary of State for Business, Innovation and Skills and Lord President of the Council.

This week, as Business Secretary, he will play a major role in leading the "national plan" for steering Britain out of recession. As Lord President of the Council, he presides over meetings of the Privy Council. He would chair meetings of the Emergency Privy Council, used in an unexpected dissolution of Parliament or, as in the case of the 2000 fuel protests, to give the Army extraordinary powers to secure petrol supplies.

This role may seem little more than ceremonial, but it was one of the many titles held by his beloved grandfather, Herbert Morrison. Morrison was also deputy prime minister and foreign secretary, two roles Lord Mandelson has yet to hold. But despite this, those who have encountered the peer recently describe his demeanour as "more than chipper".

The question on many Westminster lips is: behind this charming new guise, does the calculating and ruthless Mandelson of old still exist?

In the autumn, when Labour's rebels are expected to mount another attempt to topple Mr Brown, Lord Mandelson will be crucial in deciding whether it is all over for the PM. Perhaps an insight into what might happen can be found in a foreword he wrote to a Morrison biography in 2001: "Everyone in active politics has to be conscious of their 'sell-by' date ... In politics, people will support you for what you can do for them in the future, not for what you have achieved for them in the past. Loyalty in politics is always a complex calculus of affection and utility."