He told Jon Snow of Channel 4 News last night that he could not guarantee that another innocent man would not be shot because officers have had to assess whether a suspect is a suicide bomber on 250 occasions in the past 20 days.

Sir Ian said: “I know that there have been 250 incidents since July 7 where we have considered whether we are seeing a suicide bomber. I know . . . there have been seven times when we have got as close to calling it as that. And we haven’t. This is professional judgement.

“There are suicide bombers out there and we have a job to do. It is a tragedy but it is not a tragedy that should divert us from the main issue.”

When Mr Snow asked him if that meant that there have been seven times when an officer has come to close to shooting a suspect, Sir Ian replied: “An officer is faced with a decision as to whether to ‘call’ the particular operation, and that has happened seven times.”

Senior Metropolitan police sources clarified Sir Ian’s comments as meaning that officers have come close to being ordered to shoot a suspect dead on seven separate occasions.

His comments will dismay human rights organisations and Muslim groups angered by the shoot-to-kill policy.

Last Friday Jean Charles de Menezes, 27, a Brazilian electrician, was shot dead at Stockwell Underground station in South London.

Sir Ian’s comments came as passport checks were brought back for everyone leaving the country in an attempt to prevent the four suspects in the failed bomb attacks in London from fleeing abroad.

The controls were imposed after the first wave of bombings on July 7 but were lifted again.

Last night questions were asked about the decision to lift passport checks on people leaving the country when police were warning that a second attack was likely.

Passengers on cross-Channel ferries at the weekend also complained that people were able to enter the country without any searching of bags.

David Archer, a company director from Streatham in southwest London, said that he had travelled by ferry from Calais to Dover on Sunday.

He said: “No one on board the ferry checked our baggage. I had a backpack and could have come in with a load of stuff. It was a complete farce that there was no checking.”

Mr Archer, 38, said that the back door had been left open to terrorists. “This is almost like an open invite,” he said. “I want our country to be safe. Surely that includes protecting the country against the import of explosives, biological or chemical compounds.”

A spokeswoman for SeaFrance said that until two days ago all foot passengers were having their bags checked. She said: “Maybe they decided to stop on Sunday because of some information from security but they had been carrying out total checks.” She said that she could not speak on behalf of the harbour board in Calais or at Dover.

Immigration service embarkation controls, under which everyone’s travel documents are checked on leaving the country, were reimposed after last Thursday’s attempted bombings.The SeaFrance website advises passengers to arrive early at Dover to allow time for security checks. Last night the Home Office defended the decision to lift the controls only ten days after the initial terrorist attacks. It said that the controls were imposed on July 7 at the request of the police and lifted on July 17.

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