The identities of 42 dead children whose names were assumed by undercover police officers will not be revealed to their relatives, according to a report.
The Metropolitan Police offered a general apology for the “shock and offence” the practice had caused.
But Commissioner Sir Bernard Hogan-Howe said revealing the identities used would endanger the officers concerned.
The senior officer who wrote the report on the 1980s practice told MPs it would not be used as a tactic today.
The report’s author, Derbyshire Chief Constable Mick Creedon, was asked to investigate in 2011 after the Guardian newspaper published allegations about the conduct of undercover officers.
He told the Home Affairs Select Committee ministers did not authorise the practice but refused to condemn the officers’ actions.
“It’s irrelevant what I think,” he said. “It is not a tactic we would use these days.
“It would feel very strange for me to criticise the actions of people 20, 30, 40-years-ago without knowing what they faced at the time.”
Earlier this year, the Guardian reported that officers had stolen the identities of about 80 children who died at an early age.
Mr Creedon’s report concluded that at least 42 children’s identities had, either definitely or very probably, been used by the Metropolitan Police’s Special Demonstration Squad (SDS) and its National Public Order Intelligence Unit (NPOIU).
The earliest known use of the tactic occurred between 1976 and 1981 and it was phased out from 1994 in the SDS, the report added.
But it also found that the practice might have been used by the NPOIU as recently as 2003, and that it was “highly possible” that its use was more widespread than currently understood.
The report said: “A range of officers at different ranks and roles have been interviewed by the investigation team. The information provided corroborates totally the belief that, for the majority of the existence of the SDS, the use of deceased children’s identities was accepted as standard practice.”
Sir Bernard said 14 families had contacted the Met to ask whether the identities of their relatives had been used by undercover officers.
The Met had apologised to them, and to another family that had heard separately that it might be affected by the revelations, he said.
“Undercover officers are brave men and women” and maintaining their anonymity is “vital”, Sir Bernard said.
He explained: “There are criminals behind bars and at large today who would have no qualms in doing serious harm if they discovered a former close confidant had been working for the police.
“That’s why undercover officers spent so much time building up their ‘legend’ or false identity, and why that identity must be protected forever.”
Sir Bernard added: “I believe the public do understand the necessity for police and others to do things like this to protect against a much greater harm. It was never intended or foreseen that any of the identities used would become public, or that any family would suffer hurt as a result.
“At the time this method of creating identities was in use, officers felt this was the safest option.”
But Jules Carey, a solicitor acting for Barbara Shaw, who is concerned that her son Rod Richardson’s identity was used, said: “What we heard this morning was not an apology but a PR exercise.
“The families of the dead children whose identities have been stolen by the undercover officers deserve better than this.
“They deserve an explanation, a personal apology and, if appropriate, a warning of the potential risk they face, in the exceptional circumstances, that their dead child’s identity was used to infiltrate serious criminal organisations.
“The harvesting of dead children’s identities was only one manifestation of the rot at the heart of these undercover units which had officers lie on oath, conduct smear campaigns and use sexual relationships as an evidence-gathering tool.”
He added: “Ms Shaw has told me that she feels her complaint has been ‘swept under the carpet” and she has instructed me to appeal this outcome.”
16 July 2013 Last updated at 16:29 GMT
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