An Independent Police Complaints Commission (IPCC) report has found that Commissioner Sir Ian Blair tried to prevent its investigation into the death of Brazilian Jean Charles de Menezes in 2005.
The report underlined a series of failings, including poor communication between officers, that led to Mr de Menezes being wrongly identified as a terrorist.
IPCC chair Nick Hardwick said in a statement: "The commissioner attempted to prevent us carrying out an investigation. In my view, much of the avoidable difficulty the Stockwell incident has caused the Metropolitan Police arose from the delay in referral.
"In June 2006, the regulations were changed to put beyond doubt the IPCC's powers to investigate an incident of this kind. Other concerns about the post-incident procedures still have to be resolved.
"Very serious mistakes were made that could and should have been avoided. But we have to take the utmost care before singling out any individual for blame," Hardwick added.
Prior to the report, London's legislative assembly called on Metropolitan Police chief Sir Ian Blair to accept responsibility for endangering the public during the fatal shooting of De Menezes in 2005. And again, Sir Blair refused to resign.
"In so far as I have had sight of the report for over 18 months and have been well aware of its contents, its publication today makes no difference to the position I made clear yesterday - I intend to remain in this post," he said.