MI5 is being accused of a cover-up for failing to disclose to a parliamentary watchdog that it bugged the leader of the July 7 suicide bombers discussing the building of a bomb months before the London attacks.

MI5 had secret tape recordings of Mohammad Sidique Khan, the gang leader, talking about how to build the device and then leave the country because there would be a lot of police activity.

However, despite the recordings, MI5 allowed him to escape the net. Transcripts of the tapes were never shown to the parliamentary intelligence and security committee (ISC), which investigated the attacks.

The disclosures prompted allegations of a “whitewash” from politicians and victims of the attacks this weekend.

Last week the committee, whose members are appointed by Tony Blair and report to him, cleared MI5 of blame after it failed to thwart the attacks, which killed 52 innocent people and injured more than 700. It concluded that MI5 had no reason to suspect Khan of plotting attacks in Britain. He was regarded as “peripheral” to higher priorities.

The new evidence shows MI5 monitored Khan when he met suspects allegedly planning another, separate attack; that he had knowledge of the “late-stage discussions” of this plot; and that he was recorded having discussions with them about making a bomb and leaving the country. He was also recorded talking about his plans to wage jihad — holy war — and go to Al-Qaeda terrorist camps abroad.

Yesterday David Davis, the shadow home secretary, said: “If this is true, it completely undermines the basis on which the ISC did its report.”

Patrick Mercer, the Tory spokesman on homeland security, said: “Unless there is a proper independent inquiry, there is a danger of the committee’s report being interpreted as a whitewash.”

A committee member, who asked not to be named, admitted that it had not seen transcripts of MI5’s recordings of Khan. Instead, it had taken evidence from senior security officials and accepted their judgment that there was no reason to regard Khan as a serious threat.

The MP said that if the transcripts showed Khan had been involved in discussions about bomb-making and another possible attack, the committee had been seriously misled. “If that is the case, it amounts to a scandal,” said the source. “I would be outraged.”

Rachel North, a survivor of the bomb at King’s Cross, was shocked by the disclosure: “I am shaking with anger.

In the absence of an independent inquiry answering the public’s questions, I had hoped that those who heard the evidence behind closed doors on our behalf would find out the answers for us.

“They did not find out nor tell us the whole truth, and I feel badly, desperately let down.”

The disclosures will increase pressure for a public inquiry into the atrocity, with greater powers to demand evidence and interrogate witnesses.

The government also failed to address concerns about what MI5 knew when they were raised in unreported exchanges in the Commons last week. Davis referred to the existence of the tape recordings when he addressed John Reid, the home secretary.

“It seems that MI5 taped Mohammad Sidique Khan talking about his wish to fight in the jihad and saying his goodbyes to his family — a clear indication that he was intending a suicide mission . . . he was known to have attended late-stage discussions on planning another major terror attack. Again, I ask the home secretary whether that is true.”

Reid said the questions were “legitimate” but failed to answer them.

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