Smartphone users can do "very little" to stop security services getting "total control" over their devices, US whistleblower Edward Snowden has said.
The former intelligence contractor told the BBC's Panorama that UK intelligence agency GCHQ had the power to hack into phones without their owners' knowledge.
Mr 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.
Mr Snowden spoke to Panorama 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.
Mr 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.
"Nosey Smurf is the 'hot mic' tool. For example if it's in your pocket, [GCHQ] can turn the microphone on and listen to everything that's going on around you - even if your phone is switched off because they've got the other tools for turning it on.
"Tracker Smurf is a geo-location tool which allows [GCHQ] to follow you with a greater precision than you would get from the typical triangulation of cellphone towers."