Business secretary concedes prime minister is 'very demanding' and 'impatient' but says country needs strong leader

Lord Mandelson today defended Gordon Brown after damaging revelations in today's Observer that his abuse of his staff got so bad it led to a reprimand from the head of the civil service, but conceded that the prime minister does get furious with those around him.

A new book by the Observer's political commentator Andrew Rawnsley reveals the cabinet secretary, Sir Gus O'Donnell, launched his own investigation into reports of Brown bullying his staff.

The claims, including an account of how Brown seized his deputy chief of staff by the lapels, have been fiercely denied by Downing Street and the cabinet office.

However, the business secretary conceded there was a "passionate" and "emotional" side to Brown's character that people didn't see. "He gets angry but chiefly with himself," Mandelson said. "He doesn't so much bully people as he is very demanding. He's demanding of himself and people around him. He does not like taking no for an answer."

Speaking on BBC One's Andrew Marr show, Mandelson said: "Yes there's a degree of impatience about the man, but what would you like? Some kind of shrinking violet at the helm of the government when we are going through such stormy waters?"

Mandelson appeared to admit he had been subjected to angry tirades from Brown. Asked if he had ever been hit by Brown, Mandelson said: "History records that we had our moments, but I would like to think that I took my medicine like a man."

Extracts of the book, which is being serialised by the Observer, detailing a series of alleged furious outbursts have again raised questions about the prime minister's character.

William Hague, the shadow foreign secretary, said Rawnsley's claims were "part of a pattern of allegations about the prime minister's judgment and behaviour".

Speaking to Sky News, Hague said: "I don't think he [Brown] has ever shown that he can lead a happy team and a successful team and maybe if there's any truth in these allegations that's part of the reason why."

Alan Johnson, the home secretary, said he didn't recognise Rawnsley's portrayal of the prime minister. "I have got no knowledge of him effing and blinding at officials," he told the BBC's Politics Show. He said he was struck by how "calm and softly spoken" Brown remained in so many "fraught and pressurised situations."

The book suggests Sir Gus was so concerned at reports he received of Brown's anger that he investigated before delivering a "pep talk" to the prime minister and a warning to calm down.

A cabinet office spokesman said: "It is categorically not the case that the cabinet secretary asked for an investigation of the prime minister's treatment of No 10 staff. These assertions have been put to the cabinet secretary, who has rejected them."

Mandelson said he believed Sir Gus's version of events. But speaking on Sky News, Rawsnley stood by the allegations, which he insisted were "impeccably sourced". He said: "I was particularly careful in this area because it's obviously a very sensitive area to be sure that this wasn't gossip or hearsay or tittle-tattle. The sources were 24-carat. Not third-hand or second-hand [but] first-hand sources."

Rawnsley also said he was "bewildered" why Brown choose to deny that he had hit anyone, an allegation that was not in the book.

In an interview with Channel 4 News yesterday, Brown said things were sometimes said "in the heat of the moment", but added: "I have never hit anyone in my life."

The prime minister's official spokesman said: "These malicious allegations are totally without foundation and have never been put to No 10."

According to Rawnsley's book – titled The End of the Party and based on interviews with "hundreds" of ministers and officials – Sir Gus felt the need "to calm down frightened duty clerks, badly-treated phone operators and other bruised staff" and tell them not to "take it personally".

Brown, the book claims, turfed a typist out of her seat to take over the keyboard, swore at senior aides and "roughly shoved aside" one adviser.

In another incident, the prime minister was said to be so incensed at being given unwelcome news that he thumped the back of a protection officer's car seat so hard that he flinched, while the aide who delivered the news feared Brown "was about to hit him in the face".

The revelations emerged hours after Brown unveiled Labour's election campaign at a rally at the University of Warwick. They risk undermining recent efforts to portray a softer side of the prime minister, such as his tearful television interview with Piers Morgan.