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The secret surveillance programme has a specialist internet and phone-tapping centre at MI5's headquarters in Vauxhall, central London, according to IT website, The Register, and its veteran espionage journalist Duncan Campbell.

Mr Campbell wrote on The Register: "The 'Big Brother' comprehensive national database system feared by many MPs has been built behind their backs over the last decade."

The report comes two months after Snowden told the BBC's Panorama programme how GCHQ – the massive spying centre near Cheltenham – had the power to hack into smartphones without the owner's knowledge.

Mr Snowden said GCHQ could gain access to a handset by planting malware and then sending it an encrypted text message which would take pictures and listen in on its owner without them even being aware.

(Image: PA)

Mr Snowden even said the bugs were referred to as "Smurfs" after the famously noisy Belgian cartoon characters.

The UK government declined to comment on Panorama's claims.

Regarding the latest allegations, a Home Office spokesman told the Daily Star online "We cannot comment on the work of the intelligence services."

It is reported that MI5 and GCHQ intercept FOUR MILLION calls a year.

(Image: AP)

After listening in on those calls they could install malware but had to have a warrant signed by a Cabinet minister.

The Home Secretary Theresa May has been trying to push through new laws to allow MI5 and GCHQ to snoop on anyone's internet use without the need for a warrant.

She says it is needed to combat terrorism. Last month she offered a concession – that the information could only be stored for one year.

The codename Preston has never before been revealed but sometimes prosecutors are given what is known as "Preston briefings" if there is phone-tapping evidence which suggests a defendant may be innocent.

Snowden, 31, who worked for the US National Security Agency, went on the run in 2013, leaving behind his pole dancer girlfriend in Hong Kong.

He was given asylum in Russia but is wanted in the US where he is considered a traitor by his former bosses.