Police and MI5 then identified an Al-Qaeda cell that had carried out extensive research and video-recorded reconnaissance missions in preparation for the attack.

The encrypted e-mails are said to have been decoded with the help of an Al-Qaeda “supergrass”. By revealing the terrorists’ code he was also able to help MI5 and GCHQ, the government’s eavesdropping centre at Cheltenham, to crack several more plots.

The discovery of the suspected Commons nerve gas plot was behind the decision to increase security around parliament this summer.

A senior officer said that the scheme had led to the intervention of Eliza Manningham-Buller, head of MI5, to assess parliament’s security.

The operation to deter the sarin gas attack is referred to in an internal police document obtained by The Sunday Times.

It is a minute of a meeting of senior police officers held last month at Specialist Operations 17 (SO17), the unit responsible for protecting parliament, and reveals that the team were waiting to be briefed on the plot.

This weekend a senior officer disclosed that the thwarted plot mentioned in the document involved a gas or chemical “dirty bomb” attack against parliament. “The House of Commons was one of their targets as well as the Tube,” he said.

“They were planning to use chemicals, a dirty bomb and sarin gas. They looked at all sorts of ways of delivering it.”

But despite the successful police operation and upgraded security measures, senior officers are worried that security at the houses of parliament remains “unacceptable”.

The police security memo, drawn up after the July 7 attacks, reveals high-level fears that suicide terrorists could use a black cab or a visit to an exhibition to mark the 400th anniversary of the gunpowder plot.

It discloses that a military unit — said to have been special forces — recently carried out a secret examination of security at the House of Commons. It is believed that the exercise highlighted the ease with which terrorists could kill dozens of MPs in the debating chamber.

“[It was] felt all SO17 contingency plans should be reviewed against the new threat — a plan for a Kratos [suicide bomber] incident was required,” the minutes record. A senior officer said that he “felt particular attention should be paid to cabs entering the [parliamentary] estate”.

The memo records: “[A senior official] expressed grave concern at the shortage of security officers. He was worried that commitments such as the forthcoming exhibition on the gunpowder plot just could not be covered. He felt that an unacceptable number of posts were being closed down.”

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