Although MI5 has always denied knowing that Mohammad Sidique Khan was a potential danger, the CIA is alleged to have discovered in 2003 that he was planning attacks on American cities.

The disclosures are made in a book by the award-winning author Ron Suskind that is serialised today in The Times.

The claims contradict evidence from Dame Eliza Manningham-Buller, the Director-General of MI5, to the parliamentary Intelligence and Security Committee that Khan had never been listed as a terror threat before the attack that killed 52 innocent people.

A senior British security source has told The Times that they were aware of the allegations but said that they were “untrue and one of the many myths that have grown up around Khan”.

However, the disclosures will add to demands for Tony Blair to agree to a full public inquiry into intelligence lapses before the attack on July 7. Families of the victims, preparing to mark the first anniversary, are among those calling for an independent investigation to uncover all that British Intelligence was told about the suicide bombers by international security agencies.

David Davis, the Shadow Home Secretary, said: “This new information shows that there is an unarguable case for an independent inquiry that will enable us to ensure any weakness in our security and intelligence system are put right before we face any further terrorist threat.”

Ibrahim Mogra, chairman of the mosque and community affairs committee of the Muslim Council of Britain, said that the arguments for a public inquiry were now overwhelming.

“In light of this latest claim the case for a public inquiry becomes even more clear. There are things we are not being told about what our intelligence services knew, and if the US intelligence services knew something they should bear some of the responsibility for the attacks,” he said.

The parliamentary inquiry into 7/7 found that Khan and two of the other suicide bombers were known in some form to MI5. It said that Khan was regarded as a peripheral figure.

However, Suskind, in his book The One Percent Doctrine, says that CIA agents found evidence that Khan was in contact with Islamic extremists in the US about a plot to blow up a number of synagogues on the US East Coast. He alleges that Khan made at least two trips to America to finalise attack plans and that US security officials insisted the CIA’s Counter-Terrorist Centre shared its information with a British intelligence official in London.

The book claims that Dan Coleman, who led the FBI’s investigation into al-Qaeda, had read detailed files of Khan’s many telephone calls and e-mails, beginning in 2002, to a number of US based al-Qaeda-trained militants living in New York and Virginia.

Khan, a primary school teaching mentor from Dewsbury, West Yorkshire, is alleged to have been in contact with a student from Falls Church in Virginia, who in March was sentenced to 30 years for a plot to assassinate George Bush.

E-mail transcripts monitored by the National Security Agency (NSA) show, says Suskind, that Khan, 30, was in direct contact with Ahmed Omar Abu Ali, 24. NSA intercepts also allegedly show that Khan was in touch with the US-based extremists he later met in Pakistan.

Investigations have shown that on his arrival in the US, Khan gave a false address to immigration officials about where he was staying during his visits.

Mr Coleman, known to colleagues as The Professor because of his knowledge of US al-Qaeda sympathisers, said that Khan was “a very dangerous character” who should be closely watched. He says that he does not know if Britain acted on this warning.

The CIA claims that it had only 36 hours’ warning in March 2003 that Khan had booked a flight to New York. The FBI said that it did not have the manpower to follow Khan in the US so it placed his name on a “no-fly list” to stop him from leaving Britain, according to the book.

This year, a leading US Senator, Charles E. Schumer, commenting on newspaper reports in New York that US authorities had tipped off British Intelligence, said: “This is the British version of pre-9/11, where a country receives a generalised warning and ignores it with terrible consequences.”

Suskind told The Times: “British intelligence was certainly told about Khan in March and April 2003.

“This was a significant set of contacts that Khan had, and ones of much less importance were exchanged on a daily basis between the CIA and MI5. British authorities were sent a very detailed file.

“This demonstrates a catastrophic breakdown in communication across the Atlantic.”

The alert on Khan coincided with an order for New York police to be on the lookout for improvised chemical devices on the Subway. Khan was not linked to this alleged plot.

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