Security services have lost the ability to track a quarter of the criminals previously being monitored in the wake of the Edward Snowden revelations

A quarter of the criminals being tracked by British security services have fallen off the radar since the leak of spy secrets by former US contractor Edward Snowden, it has been claimed.

A senior intelligence source said hundreds of drug lords and people traffickers had gone to ground after being alerted to methods of detection used by GCHQ.

The UK’s electronic eavesdropping centre had also suffered a “sizeable reduction” in its ability to gather intelligence on terrorists over the past year, the source added.

They said GCHQ had been forced to reassess the safety of its staff and other people it works with in “hundreds of cases”.

Since Snowden stole and leaked 1.7m sensitive documents from the National Security Agency (NSA) a year ago, the source told the Sunday Times that GCHQ’s ability to monitor domestic and foreign crime syndicates “has reduced by about 25% over the past 12 months”.

They added that the disclosures had led criminals and terrorists to communicate in different ways, in a bid to avoid detection.

Sir David Omand, a former director of GCHQ, said the Snowden leaks had led to the shutdown of some of GCHQ’s operations.

He said: “There is no doubt in my mind that substantial damage to UK security has been caused by the Snowden revelations. I think it inevitable that many of those who have something to hide will have changed their communication methods as a result.”

However Emma Carr, acting director of the civil liberties group Big Brother Watch, questioned the accuracy of the claims.

She said: “The public isn’t told the specifics of the threat and what the revelations have done to the capability.”