MI6 has been forced to move some of its intelligence agents after Russia and China accessed secret files stolen by Edward Snowden, according to senior UK Government and intelligence sources.
Mr Snowden - the US whistleblower who leaked thousands of documents about surveillance programmes run by the National Security Agency and foreign counterparts, including Britain's GCHQ - has insisted that crucial data in the material could not be accessed by foreign powers.
But UK security chiefs have warned that his actions had compromised anti-terror activity, with former GCHQ director Sir David Omand describing it as a "huge strategic setback" for the West.
A Downing Street source said Moscow and Beijing had "cracked" information that exposed intelligence-gathering techniques and could identify individual spies.
"It is the case that Russians and Chinese have information," the source told the Sunday Times.
"It has meant agents have had to be moved and that knowledge of how we operate has stopped us getting vital information."
The source said there was "no evidence of anyone being harmed" as a result.
The revelation came with renewed focus on spy agencies' powers to collect communications data in bulk, after a review - commissioned in the wake of the Snowden revelations - said the framework was "undemocratic" and must be entirely rewritten.
Terror laws watchdog David Anderson called for security services and police to keep intrusive powers to combat terrorism and serious crime but urged the Government to draw up "comprehensive and comprehensible" new legislation - including switching authorisation from ministers to judges.
The Sunday Times quoted a senior Home Office source as saying: "(Russian president Vladimir) Putin didn't give him asylum for nothing.
"His documents were encrypted but they weren't completely secure and we have now seen our agents and assets being targeted."
A British intelligence source was quoted as saying: "We know Russia and China have access to Snowden's material and will be going through it for years to come, searching for clues to identify potential targets.
"Snowden has done incalculable damage. In some cases the agencies have been forced to intervene and lift their agents from operations to prevent them from being identified and killed."
Mr Snowden fled to Russia after his identity was revealed. He remains wanted by the US authorities.
