British spies have the ability to turn people's mobile phones off and on and switch on the handset microphone on to listen to what is happening around them, US whistleblower Edward Snowden has claimed.
The former National Security Agency analyst, a fugitive in Russia since he revealed many of the NSA's secrets, said that Government Security Headquarters (GCHQ) has a programme called "the Smurfs", after the popular Belgian cartoon characters. Its existence has also previously been reported on by Amnesty International.
As well as turning a phone on and listening in, it also allows agents to track a subject's movements with greater than usual accuracy, Mr Snowden said in an interview with the BBC's Panorama.
Mr Snowden, who now lives in Moscow, told the programme: "Dreamy Smurf is the power management tool which means turning your phone on and off without you knowing.
"Nosey Smurf is the hot mic-ing tool, so, for example, if it's in your pocket they can turn the microphone on and listen to everything that's going on around you."
He said a third tool, called Tracker Smurf, allows the phone to be tracked closely, adding: "They want to own your phone instead of you."
In the programme he describes GCHQ as "for most intents and purposes a subsidiary of the NSA", using its technology and following its guidance.
In the programme, Mr Snowden denied claims by Mark Giuliano, the deputy director of the FBI, that he is a "traitor" and said he would be willing to return to the US and go to prison.
"The question is, if I was a traitor, who did I betray? I gave all of my information to American journalists and free society generally."
The UK Government claimed in June that Britain had been forced to withdraw intelligence agents from operations because Russia and China had obtained access to secret information in files stolen by Mr Snowden
He triggered a wave of controversy when he leaked tens of thousands of documents about surveillance programmes run by the NSA and foreign counterparts, including GCHQ, in 2013.
He fled to Hong Kong where he met journalists to co-ordinate a series of articles that exposed mass surveillance programmes such as the NSA's Prism and GCHQ's Tempora, which involve "hoovering up" vast volumes of private communications.
Once his identity was revealed he fled to Russia, and he remains wanted by US authorities.
Asked if he would be prepared to do a deal with US prosecutors he said: "Of course, I've volunteered to go to prison with the government many times. What I won't do is I won't serve as a deterrent to people trying to do the right thing in difficult situations."
He added: "I regret that I didn't come forward sooner because the longer you wait with programmes like this, the more deeply entrenched they become.
"I have paid a price but I feel comfortable with the decisions I've made. If I'm gone tomorrow, I'm happy with what I had, I feel blessed."
A GCHQ spokeswoman said: "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."
:: Panorama - Edward Snowden: Spies And The Law, is being shown on BBC One at 8.30pm tonight.