Clarke will testify in the inquiry into the 11 September attacks

His fourth boss, George W Bush, may be regretting the choice.

Mr Clarke has turned on his former master, a year after stepping down as the cyber-security adviser charged with protecting America against an "electronic Pearl Harbour".

He has accused President Bush of doing a "terrible job" fighting terrorism - of ignoring the al-Qaeda threat before 11 September 2001 and distorting it afterwards.

His comments coincided with the publication of his book, Against All Enemies - a scathing account of his tenure under Mr Bush.

White House officials have moved swiftly to limit the damage, dismissing Mr Clarke's assault as politically-motivated pre-election spin.

They can take heart from his past - a career showing him to be no stranger to controversy and clashes with superiors.

But with 30 years of government service behind him, Mr Clarke is also a survivor - a man whose expertise cut across party boundaries and a voice few presidents could afford to ignore.

Israel weapons row

Richard Clarke rose to prominence in the Reagan administration of the 1980s, when he became the second-ranking intelligence officer in the State Department.

According to the New York Times, he was credited with devising methods of psychological warfare against the Libyan leader, Colonel Muammar Gaddafi.

He left the State Department in 1992 - then serving in the administration of George Bush senior - amid a bitter row over Israel's alleged misuse of American military technology.

A State Department inspector accused him of going against the US government line by turning a blind-eye to Israel's sale of weapons bought from the US to China.

Mr Clarke rebutted the charge, saying it had been fully investigated.

Al-Qaeda strike

Next, President Bill Clinton appointed him to head a committee of top officials from the FBI, CIA, the Justice Department and the US military.

The US believed this Khartoum factory made weapons for al-Qaeda

In regular top-secret meetings, the officials weighed up the threats American interests faced in a post-Cold War environment - namely terrorism and narcotics.

Mr Clarke became one of the first US officials to initiate military action against al-Qaeda when, long before 11 September 2001, he argued for cruise-missile strikes against a target in Sudan.

Later reports suggested that the bombed target - premises apparently being used by Osama Bin Laden to produce chemical weapons - was, in fact, a medicine factory.

Faulty intelligence was blamed.

'Bureaucratic guerrilla'

Mr Clarke was one of the few top officials from the Clinton era to be retained by George W Bush's administration, which brought him into the National Security Council.

After the 2001 attacks on Washington and New York, Mr Clarke was criticised for discussing intelligence failings in the press.

"Clarke also screwed up. He was after [all] the counter-terrorism tsar when 9/11 took place," Vince Cannistraro, former chief of operations at the CIA's Counter-terrorism Centre, told Computer World magazine in January 2003.

He described Mr Clarke as "a hands-on bureaucratic guerrilla" famed for a gung-ho approach.

"He was contemptuous of the bureaucracy and this attitude earned him few friends," Mr Cannistraro said.

But many of Mr Clarke's critics have also credited his worth as a determined man-of-action.

Former colleagues remember a man fiercely loyal to those who worked under him - if not necessarily to his superiors.