Exclusive The "Big Brother" comprehensive national database system feared by many MPs has been built behind their backs over the last decade, and even has a name for its most intrusive component: a central London national phone and internet tapping centre called PRESTON.

PRESTON, which collects about four million intercepted phone calls a year, has also recently been used to plant malware on iPhones, according to disclosures by former NSA contractor Edward Snowden. The phones were then targetted for MI5 "implants" (malware), authorised by a ministerial warrant.

The location and role of the PRESTON tapping centre has never previously been publicly identified, although published Crown Prosecution Service guidance to senior prosecutors refers to secret "Preston briefings" which they can be given if tapping evidence in a case they are prosecuting reveals that a defendant may be innocent. (The guidance also notes that the briefing may be given after exculpatory intercept evidence has been destroyed.)

Located inside the riverside headquarters of the Security Service, MI5, in Thames House, PRESTON works alongside and links to massive databases holding telephone call records, internet use records, travel, financial, and other personal records held by the National Technical Assistance Centre (NTAC), a little known intelligence support agency set up by Tony Blair's government in a 1999 plan to combat encryption and provide a national centre for internet surveillance and domestic codebreaking.

Soon after, the Parliamentary Intelligence and Security Committee were told that the spy agencies would fund NTAC as "a twenty-four hour centre operated on behalf of all the law enforcement, security and intelligence agencies, providing a central facility for the complex processing needed to derive intelligence material from lawfully intercepted computer-to-computer communications and from lawfully seized computer data ... The NTAC will also support the technical infrastructure for the lawful interception of communications services including Internet Services."

The Home Office then commissioned and funded a technical plan to establish an interception network for the domestic internet, and allocated a £25m budget to get NTAC started.

In 2002, the Home Office announced that NTAC would continue to support the needs of law enforcement for a continuing flow of intelligence and evidence. Lingering concerns about NTAC's full planned role were shrugged off and forgotten after the 9/11 attacks.