British intelligence has had to pull agents from operations in “hostile countries,” after Russia and China obtained sensitive information by cracking over 1 million encrypted files leaked by U.S. whistleblower Edward Snowden, according to a report from the U.K.'s Sunday Times.

The report cites unnamed senior officials from the British Prime Minister's office, the country's Home Office and security services, saying that the hacked files contained details of secret intelligence techniques and information that could allow British and U.S. spies to be identified.

As a result, the officials reportedly claim, agents from the U.K.'s Secret Intelligence Service, also known as MI6, which is tasked with advancing British interests overseas, has had to remove agents from the field, in order to prevent them being identified and killed,

The paper quoted a “senior Home Office source” as saying: “Putin didn’t give [Snowden] asylum for nothing. His documents were encrypted but they weren’t completely secure and we have now seen our agents and assets being targeted.”

It also quoted an a “British intelligence source” as saying that “Snowden has done incalculable damage”. However an official at the Prime Minister's office was quoted as saying that there was "no evidence of anyone being harmed."

Another senior U.K. government source told the BBC that the information obtained by Russia and China meant that "knowledge of how we operate" had stopped the U.K. getting "vital information".

Snowden, a former U.S. intelligence contractor, leaked millions of classified documents to the media in 2013. The information made public the extent of a previously unknown NSA electronic surveillance program, which has since been curtailed.

The U.S. wants Snowden to be tried for his actions, but he has been granted asylum in Russia, and has not expressed a willingness to return to the U.S. to face legal proceedings.

In the aftermath of the Snowden revelations, U.S. officials branded the damage done to the country's intelligence capabilities as “grave,” adding that the scope of information revealed had been “staggering”.