THE death of radical Islamist cleric Anwar al Awlaki, regarded as al Qaeda's most potent threat to the West, is a significant blow to the terrorist group, Foreign Secretary William Hague has said.

Awlaki, said to be leader of external operations with the group’s most active operational affiliate, al Qaeda in the Arabian Peninsula (AQAP), and once tipped as Osama bin Laden’s successor, is believed to have been one of four people killed in an air strike at 9.55am yesterday in the town of Khashef in Jawf province in Yemen.

Mr Hague said Awlaki, a dual US-Yemeni citizen, had “demonstrated his intent and ability to cause mass terror, while his murderous ideology was responsible for inspiring terrorist attacks in the UK and the US”.

He warned: “We must keep up the pressure on al Qaeda and its allies and remain vigilant to the threat we face.”

It is understood the strike on Awlaki was directed by the same CIA and US Joint Special Operations Command team that led the Osama bin Laden assassination. Awlaki has been on the run in Yemen since December 2007 and was wanted dead by US President Barack Obama for his links to a host of terror plots. Mr Obama said last night that AQAP was now “a dangerous but weakened terrorist organisation”.

Yemen’s defence ministry statement said only that Awlaki had died in Khashef “along with some of his companions”.

US and Yemeni officials later named one of those as Samir Khan, also a US citizen but of Pakistani origin, who specialised in computer programming for al Qaeda and produced the group’s English-language online magazine, Inspire.

Tribal leaders said Awlaki had been moving around Yemen to evade capture and locals claimed he had been between Jawf and Marib provinces when he died.

It was reported in November last year that Awlaki, said to have been one of the top terrorist recruiters because of his fluency in English and use of technology, radicalised a generation of young Muslims during a “grand tour” of Britain from London to Aberdeen, as part of a campaign by the Muslim Association of Britain.

The so-called “bin Laden of the internet”

had been fleeing an FBI inquiry in America in the wake of his involvement with three of the September 11 hijackers, when he arrived in Britain in 2002. But unable to support himself he returned to Yemen in 2004.

Awlaki, 40, who was born in the US state of New Mexico in 1971 to Yemeni parents, is suspected of inspiring the mass shooting at Fort Hood army base in Texas in 2009 and of taking a role in planning the attempted suicide bombing of an airliner heading for Detroit on Christmas day 2009.

He is believed to have links to the September 11 attacks, and the foiled East Midlands Airport cargo plane bomb plot in November last year.

His preachings also inspired Roshonara Choudhry, the woman convicted in November last year of trying to murder Stephen Timms, the former Labour Cabinet Minister, in his London constituency office.

Mr Obama described the killing of Awlaki as “a major blow to al Qaeda’s most active operational affiliate” and vowed to prevent the terror network from finding a haven.

He said: “The death of Awlaki marks another significant milestone in the broader effort to defeat al Qaeda.”

American officials said Awlaki was targeted in the killing, but Khan, who edited the slick Jihadi internet magazine, was apparently not targeted directly. Khan, who was from North Carolina, was not considered an operational leader but had published seven issues online of Inspire which offered advice on how to make bombs.

Mr Obama praised Yemen’s government and security forces for their close co-operation with the US in fighting AQAP, said to be the terror network’s most dangerous affiliate.

Last year Awlaki’s father filed a court action against Obama, then-CIA chief Leon Panetta and then-Secretary of Defence Robert Gates to prevent the US government from targeting his son for assassination.

A judge threw out the case last December, saying a judge does not have authority to review the President’s military decisions and that Awlaki’s father did not have the right to sue on behalf of his son.

Civil liberties groups questioned the killing of an American without trial.

The American Civil Liberties Union said the Awlaki killing was part of a counterterrorism programme that “violates both US and international law”.

Sajjan Gohel, of the international policy assessment group the Asia Pacific Foundation, said: “Al-Awlaki didn’t need subtitles to indoctrinate. He understood how to impact the Muslim diaspora in the West.”