The former intelligence contractor said that UK intelligence agency GCHQ had the power to hack into phones without their owners’ knowledge.

Snowden said GCHQ could gain access to a handset by sending it an encrypted text message and use it for such things as taking pictures and listening in.

The UK government declined to comment.

Snowden spoke to the BBC in Moscow, where he fled in 2013 after leaking to the media details of extensive internet and phone surveillance by his former employer, the US National Security Agency (NSA).

He did not suggest that either GCHQ or the NSA were interested in mass-monitoring of citizens’ private communications but said both agencies had invested heavily in technology allowing them to hack smartphones. “They want to own your phone instead of you,” he said.

Snowden talked about GCHQ’s “Smurf Suite”, a collection of secret intercept capabilities individually named after the little blue imps of Belgian cartoon fame.

“Dreamy Smurf is the power management tool which means turning your phone on and off without you knowing,” he said.