Intelligence agency GCHQ is finding it impossible to track some of Britain’s most dangerous criminal gangs because their tactics were disclosed in The Guardian. The intelligence agency, which monitors communications, has claimed criminal gangs changed their tactics after Edward Snowden leaked information about how they are monitored.

In addition to criminal gangs changing their communication methods, GCHQ is also unable to monitor groups in the way they used to as it is now much easier to spot them. Gangs now know what to look for, and “go dark” if they know they are being monitored. As a result any method that can be detected is not being used for fear of exposing the investigation.

Ever since Snowden handed over the documents there has been significant discussions about how his actions have affected diplomacy and counter-terrorism. This is the first time the security services have admitted the damage the leaks have had on criminal investigations.

GCHQ work with the National Crime Agency (NCA) to gather evidence against Britain’s most dangerous criminals including major drugs gangs, paedophile rings and human traffickers. The gangs involved are often sophisticated and have the resources to change their communications methods based on the information given to the Guardian by Snowden.

A senior security official told the Daily Telegraph: “Snowden has been very damaging to our work. We have specific evidence of where key targets have changed their communication behaviour as a direct result of what they have read.

“They have moved to more secure forms of communication and we have been unable to assist the NCA. It takes longer to help law enforcement and because we only focus on the most serious, the top end networks, then the impact they have in the mean time is multiplied.”

Officers have cited the example of a drugs gang that changed their communications methods after the Guardian articles. Although there is no reason to suspect they knew they were being monitored they nonetheless used the information to improve their own security. GCHQ believe the gang has not been broken up because the monitoring of it has been disrupted.

They are now forced to be much more careful: “We have techniques that need to be protected… The choice is not to pursue a network and we have decided not to press ahead where there is a possibility of being detected,” the source said.