Mandelson: I’m only here so Gordon can enjoy his holiday

As the peer swept into Whitehall from his own break in Corfu, he also confirmed that his week deputising for the Prime Minister would be very different from Harriet Harman's recent stint at the helm.

The Business Secretary said that there had been a "ridiculous song and dance about who's in charge", but suggested that he would be sticking to his ministerial brief — unlike Ms Harman.

The deputy Labour leader ran into controversy in the past fortnight as she blamed men for the financial crisis and dug her heels in over rape laws.

Lord Mandelson stressed that he would focus on plans to boost the economy and help the motor industry.

Mr Brown finally began his own family holiday in the Lake District today, although Lord Mandelson repeatedly insisted that the Prime Minister was still in overall charge of the nation.

He said: "It's been really good to go away and have nice peace and quiet on holiday. It was good and I am very glad to be back.

"If there are some small things I can pick up so he can have the best holiday he deserves then obviously I will do that. I'm not in charge of the country, the PM is in charge of the country and he has been throughout."

His return coincided with an interview in which he admitted he had for years been Labour's "hit man" but now saw himself as a "kindly pussy cat".

Downing Street today confirmed that Lord Mandelson would be dividing his time between his offices in the Department for Business, Innovation and Skills, and the Cabinet Office during his week in charge.

No 10 declined to disclose details of what Mr Brown and his family would be doing. "I am sure the Prime Minister will be engaged in a wide range of activities," said a spokesman.

Lord Mandelson will stand in for the Prime Minister from today until 16 August, when he is expected to hand over to Chancellor Alistair Darling.

The First Secretary of State, widely seen as Mr Brown's de facto deputy, was pictured at the weekend enjoying the hospitality of billionaire financier Lord Rothschild and his son Nat.

In an interview with the Guardian, Lord Mandelson described himself as more of a "kindly pussy cat" than a "big beast" of politics.

"I don't really see myself as a big beast. More as a kindly pussy cat. Yes, a kindly pussy cat. I'm a kindly pussy cat, with strong views about what we need to do," he said.

Lord Mandelson's holiday companions in Corfu included David Geffen, the American film producer and record executive, and interior designer Nicky Haslam. Mr Haslam told a newspaper yesterday that the peer had been "working extremely hard" during his week in the 30C sunshine. "He's always on the phone," he added.

Unlike last year, when he joined shadow chancellor George Osborne for an infamous dinner at a taverna, Lord Mandelson stayed within the grounds of the Rothschild estate.

In contrast to Ms Harman — who was criticised for straying beyond her brief during her two weeks on duty — Lord Mandelson plans to restrict his role to his departmental responsibilities.

Amid fears for the long-term future of 5,000 workers at Vauxhall's car plants, he will be in talks with both bidders for the company. With September the biggest month for new car sales, he will also trumpet the Government's scrappage scheme that offers £2,000 off the price of a new vehicle.

Lord Mandelson will also respond to the expected grim jobless figures — due out on Wednesday — and signal further moves to increase access to university, another policy within the remit of his huge Department for Business and Regulatory Reform. But given his overarching role across Whitehall, he is also likely to hold meetings in his new office in the Cabinet Office, said to be "the size of a badminton court".

In the interview, Lord Mandelson admitted, discussing Labour strife, that he had in effect operated as Tony Blair's "hit man". He said: "I think that everyone in politics wants to be liked, but I accepted too readily that it was a luxury that wasn't open to me. I had to be the hard man — and sometimes the hit man. I think 10 years ago, and also 15 years ago, I was a very hard-nosed, uncompromising figure who was manning the barricades of change in the Labour Party, and prepared to take down anything or anyone who stood in the way.

"I don't feel in that mode now. And, secondly, I've learned from experience that you can defeat people without killing them."

On the Cabinet: "I think people are aware that I'm not denying anyone their place in the sun. Older figures in government used to fear that I was endlessly plotting their downfall, or excluding them from the team, but the circumstances are completely different now."

And his future? He plays down speculation that he may become Labour leader. "I'm not anticipating any change for myself."