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When he returned to the UK it is thought he established links with Khalid Sheikh Mohammed, thought to be the "principal architect" of 9/11.

In the biographical section of a book he wrote in 1999, entitled The Army of Madinah In Kashmir, written under the alias Esa Al-Hindi, it said: "After training and spending time with the Mujahidin, he witnessed a side of Islam which cannot be found in the classrooms.

"It is the Islam that is felt by living and dying in the cause of Allah; loving Muslims and hating the enemies of Allah. Once this lifestyle was instilled in his blood, he began his career as a Mujahid."

Security sources say that he rose to become a leading al-Qaeda figure and one of the network's highest-ranking operatives in Britain.

Security sources also allege that Barot enjoyed the confidence of key figures in the terror network to such an extent that he was entrusted with the planning of major operations.

The way al-Qaeda is structured, with a disparate collection of cells loosely allied to each other by their devotion to a radical interpretation of Islam, makes it near impossible to accurately define Barot’s place in the network’s hierarchy.

But there is no doubt that he was a major player, one of its senior and most experienced terrorist planners, who possibly had connections to that elusive inner circle at the very top.

According to counter-terrorism sources, the conviction of Barot was "one of the most significant steps in the fight against terrorism in the UK since 9/11".

"He is one of the most senior well-trained and ruthless terrorists that we have dealt with," a source said.

"His career as a terrorist stretches back to at least 1995, well before 9/11, the invasion of Afghanistan, or indeed the invasion of Iraq.

"This is one of the first occasions that the British public will be able to see for themselves the scale of the threat we face.

"His plans, had he not been arrested, would have caused the death of hundreds if not thousands of innocent people. His arrest and conviction means that al-Qaeda has lost an experienced, utterly dedicated and vicious terrorist."

Security sources believe that Barot was probably the person identified in the 9/11 Commission’s report as Issa al-Britani, which was an alias he often used.

According to the report, Khalid Sheikh Mohammed, the principal architect of 9/11, claimed that he ordered al-Britani to Kuala Lumpar in Malaysia to learn about the jihad from notorious terror-plotter Hambali, the alleged mastermind of the Bali bombings.

Sheikh Mohammed also claimed to have sent al-Britani to New York to check out potential targets there at the behest of Osama bin Laden, the al-Qaeda leader.

By the time Barot appeared on the radar of Western intelligence agencies he had been involved in terrorism for several years and was already a highly skilled operator, adept at evading surveillance or capture.

He was extremely well organised and clearly had considerable, independent financial backing. He was trusted by the al-Qaeda leadership to conduct their research and present the results back in Pakistan.

It was not until 2004 that Scotland Yard’s Anti-Terrorist Branch and Britain’s intelligence agencies became aware that a senior al-Qaeda operative was working within the UK, and it took several months to identify that man as Barot.

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