MI5 is alleged to have kept back material from bugging operations and data gleaned from computer analysis of the bomb cell from the Intelligence and Security Committee before its critical report last week.

Doubts have also emerged about when the identity of Mohammad Sidique Khan, the ringleader of the gang, became known to intelligence services.

The omissions have intensified calls for an independent inquiry into how Britain’s security machinery failed to thwart the bombings that killed 52 Tube and bus passengers.

David Davis, the Shadow Home Secretary, said last night that there was a need for an independent inquiry headed by a senior judge. “If it is the case that information has been withheld from the ISC then that is a scandalous situation which undermines the whole basis of the committee’s judgment and reinforces the requirement for an independent inquiry,” he said. “The issue at stake is the safety of the public and it is necessary that we do everything possible to improve the performance of our security services.”

Three areas of concern have emerged since the publication last Thursday of reports on July 7 by the ISC and the Home Office.

The ISC concluded that it was “understandable” that MI5 and other agencies had not pursued inquiries about Khan and his accomplice Shehzad Tanweer after they emerged as “peripheral” figures in a separate anti-terrorist investigation. The committee said that it had been told that there was “no evidence that they (Khan and Tanweer) were connected to planning terrorist acts”.

The ISC members did not appear to be aware of bugged conversations in which Khan discussed a bomb plot with other terrorist suspects.

The ISC also accepted MI5’s assertions that although Khan had appeared on the intelligence radar, it was not until after the July 7 bombings that he was positively identified. But the Home Office narrative of events suggests that Khan’s name was already in the intelligence records. That report reveals that Khan was identified at 23.59 on July 8 as the account holder of a credit card found at the Edgware Road bomb scene.

The Home Office account adds that “in reviewing records” the next day Khan’s name appeared as having “been picked up on the periphery of another investigation”.

There are also concerns about how much the ISC was told about Tanweer’s activities.

Sources have indicated that two weeks before July 7 he had logged on to a militant website to download information about explosives. It is believed that the website was under observation, possibly by a foreign intelligence agency.

The ISC report does not mention Tanweer’s online activities while the Home Office account claims that he was not a heavy internet user.

ISC members declined to comment but some privately expressed reservations that they may not have seen all the material relevant to their inquiry. Nick Clegg, the Liberal Democrat Home Affairs spokesman, said that it would be “extraordinary” if MI5 and other agencies had not revealed everything to the watchdog committee. He said: “The ISC report was always rather limited in its remit, but frankly I don’t think we had any idea that it was as limited as it now appears to be. The Government’s position on resisting an inquiry is becoming unsustainable.”

Security sources insisted last night that all material asked for by the ISC had been handed over and claimed that there was no surveillance material to indicate that Khan was caught up in any terrorist plotting.

But Crispin Black, a former government intelligence officer, said that he was concerned that members of the ISC had been working “blind”, relying on the security services to tell them what they needed to know but not coming up with penetrating questions.

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