Newly-released letters reveal why the Met Police commissioner delayed the start of an independent investigation into a fatal shooting by police.
Sir Ian has said he will not step down over the Menezes case
In a letter to the Home Office, Sir Ian Blair says he refused the Independent Police Complaints Commission access to the crime scene, at Stockwell Tube, where Jean Charles de Menezes died.
He says the Met's counter-terrorism investigation had to take priority.
The Home Office released the letter under the Freedom of Information Act.
Mr Menezes, a 27-year-old electrician, was shot dead the day after the failed 21 July bombings in London.
Anti-terrorism police officers shot the Brazilian eight times at Stockwell station after mistaking him for a suicide bomber.
The investigation into the shooting "will be carried out by the Met's own Directorate of Professional Standards", Sir Ian writes.
"This investigation will be rigorous but subordinate to the needs of the counter-terrorism operation.
There is much concern about revealing either the tactics that we have and/or the sources of information on which we are operating
Met Police commissioner Sir Ian Blair
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"In a fast-moving, multi-site terrorist situation, in which suicide bombers are clearly a very strong possibility, a chief officer of police should be able to suspend... [the part of the] Police Reform Act 2002 which requires us to supply all information that the Independent Police Complaints Commission may require," he adds.
The IPCC's "duty to provide as much information as it can to the complainant or to members of the deceased's family... could put further lives at risk in these circumstances", Sir Ian says.
"There is much concern about revealing either the tactics that we have and/or the sources of information on which we are operating."
The letter ends: "Clearly, this is a developing situation but for the time being I seek your support for this measure, which may form the basis for amending legislation in the future."
Sir Ian says he is not trying to "exempt" his officers from investigation - "and, ultimately therefore prosecution, if evidence of deliberate malfeasance was available".
But he adds "prosecuting authorities" should "take cognisance of the pressures under which the service operates in terrorist scenarios".
Mr Menezes was shot a day after the failed London bombings
"This is clearly a fast-time decision-making process, in which officers cannot risk the kind of containment and negotiation tactics which would normally be the case.
"Put simply, the only choice an officer may have may be to shoot to kill in order to prevent the detonation of a device.
"In due course, I believe we need a document similar to the military rules of engagement but time does not permit its creation at the present time."
Sir Ian also says he had "raised the issue of maximising the legal protection for officers who had to take decisions in relation to people believed to be suicide bombers" with Prime Minister Tony Blair the previous day.
IPCC deputy chairman John Wadham told BBC News he was confident the independent investigation had not been damaged by the delay in referring the case.
But he added the leak to ITV News of documents related to the investigation and photographs of Mr Menezes on the train where he was shot had damaged the organisation's relationship with the police and the public.
Describing the leak as "embarassing", Mr Wadham said it was a "collective failure" that went to the "heart" of people's confidence in the IPCC.
The investigation is due to end before Christmas.
Last month Sir Ian told BBC News the case had been referred after the issues raised in his letter had been discussed at a "series of meetings" between the Met, the Home Office and the IPCC.
"We went through the appropriate processes in a unique situation," he added.
"If one was going to cover something up, you would not cover it up by writing a letter delivered by hand to the permanent secretary at the Home Office, the chairman of the Metropolitan Police Authority and the chairman of the Independent Police Complaints Commission itself."