Information relating to three judge-led inquiries, including two fatal police shootings, has gone missing in the post, the Ministry of Justice (MoJ) said.
Officials realised two discs containing documents about the Mark Duggan, Azelle Rodney and Robert Hamill inquiries were lost in early January.
It a statement the MoJ said police have taken “necessary steps” to ensure the protection of any officers whose information could be disclosed.
The police marksman who killed Mr Duggan in north London in 2011, sparking nationwide riots, has never been made public, along with the identities of other officers involved in the case.
Meanwhile, the Government has launched an investigation into the security breach of its guidelines and said disciplinary action will be taken if appropriate.
A statement from the MoJ said: “Government officials became aware on January 8 that two discs containing documents relating to these inquiries were missing, having been dispatched by post.
“Immediate steps were taken, including intensive searches to locate the discs. These searches continue, with police assistance. The discs have not, as yet, been found.”
It added: “At this stage there is no evidence to indicate that the information loss arose from malicious intent.
“Nevertheless, it is essential to take the most precautionary view and to take all necessary steps to safeguard the interests of anyone whose information could be disclosed.
“Police and other agencies have undertaken their own risk assessment, and have identified and taken any steps necessary to ensure the protection of officers.
“The Rodney, Hamill and Duggan families and the three judges who conducted the inquiries and inquest have been informed. So too has the Information Commissioner’s Office.”
Lawyers are undertaking a review of the missing documents to discover if they held confidential or sensitive details, the MoJ said.
A Metropolitan Police spokeswoman said the force has undertaken a risk assessment of the material that was on the discs and has taken “ appropriate steps”.