Britain's electronic eavesdropping agency has gained secret access to fibre-optic cables carrying global Internet traffic and telephone calls, The Guardian said on Saturday citing documents disclosed by rogue US intelligence technician Ed Snowden.
The latest claims sparked fresh outcry from privacy campaigners and surfaced as the United States filed espionage charges against 30-year-old Snowden and asked Hong Kong -- where he has fled to -- to detain him.
The Guardian said Government Communications Headquarters (GCHQ) has started processing vast amounts of personal information -- including Facebook posts, emails, Internet histories and phonecalls -- and is sharing it with its US partner the National Security Agency (NSA).
GCHQ added however that it was "scrupulous" in its compliance with the law.
The newspaper reported that GCHQ was able to tap into and store data from the cables for up to 30 days.
"It's not just a US problem. The UK has a huge dog in this fight," Snowden told the newspaper.
"They (GCHQ) are worse than the US."
Snowden, who fled to Hong Kong in May, has since proceeded to leak details of secret US intelligence programmes to international media outlets.
The Guardian claimed Tempora had been running for 18 months and GCHQ and the NSA were able to access vast quantities of communications between entirely innocent people.
It also claimed that the intelligence-gathering directly led to the arrest and jailing of a British terror cell, the arrests of others planning acts of terror, and three London-based people planning attacks prior to the city's 2012 Olympic Games.
The Guardian said the documents it had seen showed that by last year, GCHQ was handling 600 million "telephone events" each day, had tapped more than 200 fibre-optic cables and was able to process data from at least 46 of them at a time.
The two main components of GCHQ's surveillance programme are called "Mastering the Internet" and "Global Telecoms Exploitation", the daily said, adding that the operations were all being carried out "without any form of public acknowledgement or debate".
Shami Chakrabhati, director of human rights group Liberty, expressed shock at the latest revelations.
"I'm shocked -- but not surprised. Clearly they (GCHQ) are exploiting the fact that the Internet is so international in nature," Chakrabarti told the BBC.
"And I'm pretty sad in a democracy when all that appears to be holding back the secret state is its physical and technological capability and not its ethics or a tight interpretation and application of the law."
Nick Pickes, the director of privacy campaign group Big Brother Watch, added: "This appears to be dangerously close to, if not exactly, the centralised database of all our Internet communications, including some content, that successive governments have ruled out and parliament has never legislated for.
"If GCHQ have been intercepting huge numbers of innocent people's communications as part of a massive sweeping exercise, then I struggle to see how that squares with a process that requires a warrant for each individual intercept. This question must be urgently addressed in parliament."
But the chairman of parliament's intelligence and security committee, Malcolm Rifkind, said that all agencies like GCHQ used the latest technology to gain intelligence.
"What all intelligence agencies do, not just in Britain but throughout the world ... is to obtain intelligence and they use the most modern technology in order to do that," he told the BBC.
"The crucial question is not how much data could they theoretically collect but what can they get access to -- is it an intrusion on the privacy of the citizens.
"If GCHQ wants to know the content of your or my email, they have to go through exactly the same legal procedure of getting a warrant from the secretary of state, regardless of whether they are going to intercept the emails themselves, or whether they are going to ask the NSA or someone else to do it."
A GCHQ spokeswoman said: "We do not comment on intelligence matters. Our intelligence agencies continue to adhere to a rigorous legal compliance regime.
"GCHQ are scrupulous in their legal compliance."