To illustrate their concern, agents have leaked more topsecret documents to The Sunday Times because they want a public inquiry into the “missed intelligence” leading up to the July attacks in London.

They believe ministers have withheld information from the public about what the security services knew about the suspects before the bombing of July 7 and the abortive attacks of July 21.

The documents include an admission by John Scarlett, head of SIS, the secret intelligence service (also known as MI6), that one of the July 21 suspects was tracked on a trip to Pakistan just months before the attempted bombings.

Until now it was not known that any of the July 21 suspects, who are awaiting trial, were familiar to the intelligence services. It has been disclosed that MI5 had placed two of the July 7 bombers under surveillance before their attack, but judged them not to be a threat.

The new documents show that MI5, which is responsible for national security, allowed the July 21 suspect to travel to Pakistan after he was detained and interviewed at a British airport. Once in Pakistan he was monitored by SIS, which gathers intelligence overseas.

MI5 then conducted what the leaked memo says was “a low-level short-term investigation” into the suspect, who cannot be named for legal reasons.

It stopped monitoring him because it said “the Pakistani authorities assessed that he was doing nothing of significance”.

Scarlett revealed details of the operation to the parliamentary intelligence and security committee (ISC) last November. The committee, comprising MPs and peers picked by Tony Blair, is conducting a secret inquiry into the “lessons learnt” from the July attacks. It is due to be completed in April.

The Scarlett memo — marked top secret — was leaked by the dissident officers who want a public inquiry similar to that undertaken in America after the 9/11 attacks.

They believe it would highlight the need for MI5 and SIS to be given more resources to deal with Al-Qaeda. They are critical of Blair, who has ruled out an inquiry saying it would distract the security services from fighting terrorism.

The leaked memo refers to Scarlett as C — the traditional codename for the head of SIS. It states: “On the events of July itself, and the question of whether intelligence was missed, C noted that SIS had previously been involved in an earlier investigation of one of the July 21 (suspects) in Pakistan.

“This had been at the Security Service (MI5)’s behest and should be discussed with MI5.”

Another document, MI5’s November 2005 memo The July Bombings and the Agencies’ Response, has also been shown to The Sunday Times.

It names the suspect who was the subject of the 2004 investigation and shifts responsibility for the decision to stop monitoring him to the Pakistani intelligence authorities.

“(The suspect) had been the subject of a low-level short-term investigation concerning a visit he made to Pakistan after he was interviewed on departure from the UK,” it states.

“However, the Pakistani authorities assessed that he was doing nothing of significance in a terrorist context.”

The assessment echoes a decision by MI5 to halt surveillance on two of the July 7 bombers 16 months before the attacks. Both were filmed and taped by MI5 agents as they met two men allegedly plotting to carry out a terrorist attack in England.

After making what an official called “a quick assessment”, MI5 concluded Mohammad Sidique Khan and Shehzad Tanweer were not immediate threats. As the MI5 memo puts it: “Intelligence at the time suggested Khan’s purpose was financial crime rather than terrorist activity.”

David Davis, the shadow home secretary, said: “These leaks show that the need for an independent inquiry is incontrovertible.”

There is a growing consensus in Whitehall that the intelligence services will be seen to have made critical errors in failing to assess adequately the threat from at least three of the July suspects.

Scarlett conceded to the ISC that his agency had reacted too slowly. “Summing up the position before July 2005, C noted SIS were conscious of the size of the target, but equally conscious of what we did not know; we were thinly spread in North and East Africa; we were looking at new ways of increasing our reach; and we had sought funding to grow as fast as we thought feasible.

“Turning to the lessons learnt, C noted that SIS had understood the nature of the threat and that there was a great deal that we did not know. SIS had developed strategies to meet this threat.

“The attacks had shown that our strategies were correct, but needed to be implemented more extensively and more quickly,” the memo noted.

Scarlett said that even before the attacks, SIS had planned to expand overseas. “C concluded by explaining how post-July SIS were speeding up implementation of the pre-July strategy.” He said the agency did not want more money for staff.

The dissident officers believe the buck-passing revealed in the memos demonstrates that there should be closer co-operation between the agencies.

They support calls for a unified department of homeland security, along the lines suggested by Gordon Brown, the chancellor, this month.

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