Edward Snowden has said the UK's intelligence agency has a suite of programmes that would allow it to take over our smartphones even when its off.
In an interview for the BBC's Panorama program he said the GCHQ had "invested heavily" in technology to hack smartphones belonging to the public.
"They want to own your phone instead of you," he explained.
Describing the agency's capabilities he said it possesses a "Smurf Suite" - named after the cartoon show - allowing it to easily hack mobile phones.
"Dreamy Smurf is the power management tool which means turning your phone on and off with you knowing," he told the BBC.
"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."
In the programme, titled 'Edward Snowden, Spies and the Law' he also compared the GCHQ to its US counterpart, the National Security Agency (NSA).
"GCHQ is to all intents and purposes a subsidiary of the NSA.
"They [the NSA] provide technology, they provide tasking and direction as to what they [GCHQ] should go after."
Snowden, a former intelligence contractor previously employed by the NSA was speaking to the BBC from Moscow.
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He fled there in 2013 after he leaked a tens of thousands of documents detailing mass surveillance programmes run by the NSA and intelligence agencies around the world including GCHQ.
However, GCHQ maintains that all its work is "carried out in accordance with a strict legal and policy framework."
A GCHQ spokeswoman told the Press Association: "It is long-standing policy that we do not comment on intelligence matters.
"Furthermore, all of GCHQ's work is carried out in accordance with a strict legal and policy framework, which ensures that our activities are authorised, necessary and proportionate, and that there is rigorous oversight, including from the Secretary of State, the Interception and Intelligence Services Commissioners and the Parliamentary Intelligence and Security Committee.
"All our operational processes rigorously support this position. In addition, the UK's interception regime is entirely compatible with the European Convention on Human Rights."