Buckingham Palace, the Houses of Parliament and New Scotland Yard were completely sealed during a frantic period in July 2005 when police feared an imminent suicide attack.

The security lockdown was ordered on July 12 because it was believed that the terrorist cell which carried out the 7/7 bombings might have had a fifth member who was still on the loose.

For a 90-minute period no-one was allowed to enter or leave any of the buildings, all of which were regarded as landmark targets for terrorist attack.

The detail of the operation was revealed at the beginning of the second week of the inquest into the death of Jean Charles de Menezes.

Mr de Menezes, 27, a Brazilian electrician, was shot seven times in the head 10 days after the lockdown, by police officers who had mistaken him for a suicide bomber.

Peter Clarke, the former head of the Metropolitan Police’s Counter-terrorism Command, said that the month that saw the 7/7 and 21/7 bombings was a period of intensive police activity.

Police officers were working around the clock, the threat level was placed at “severe” and intelligence suggested another attack would happen at any time.

On July 12, five days after the 7/7 attacks which had killed 52 people, police found a bomb factory in Leeds and the bombers’ car at Luton rail station.

Initial examination of the evidence and CCTV footage from the railway station suggested there might have been a fifth member of the bomb gang who was still at large.

An order was given to impose a complete lockdown on the three buildings.

“It was completely unprecedented as was some of the decision making having to be made at that time about whether to warn the public about the possibility of a fifth suicide bomber on the loose or not,” said Mr Clarke.

“If you warn the public you could cause unnecessary panic, if you don’t and something terrible happened, the obvious question is why didn’t you warn the public?

“That’s the sort of pressure we were under day in, day out at that time. The 12th July was but one example.”

Mr Clarke said he had been out of London on the day Mr de Menezes was shot because of the impact of the 7/7 attacks on his own family.

On July 7, his 16-year-old son had been due to catch a train to Cambridge but rang his parents to describe the panicky situation at the mainline station.

Suspecting, but not knowing immediately there had been a bomb underground, Mr Clarke and his wife advised their son to catch a bus and get away from the area.

An hour later a bomb exploded on a No30 bus in nearby Tavistock Square. The mobile phone network had crashed and neither parent was able to contact the boy for several hours.

Mr Clarke sent his family on a planned holiday a few days later while he remained at work.

On July 20 his wife had called in a state of distress because of “delayed shock” and Mr Clarke went abroad to see and reassure his family.

No sooner had he arrived than the 21/7 attempted bombings occurred followed by the shooting of Mr de Menezes. The officer was back at Scotland Yard by the evening of July 22.

Mr Clarke added: “This was a period of unprecedented intensity in terms of policing activity, investigation and the expectation and fear of further attacks.”

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