British spies have secretly accessed fibre-optic cables carrying huge numbers of emails, Facebook messages and other communications, according to The Guardian.
Documents given to the newspaper by US whistleblower Edward Snowden suggest eavesdropping agency GCHQ can analyse data from the network of cables that carry global phone calls and internet traffic under an operation codenamed Tempora.
The newspaper said data had been shared with the organisation's US counterpart, the National Security Agency.
GCHQ, in Cheltenham, refuses to comment on intelligence matters but insisted it was "scrupulous" in complying with the law.
It is the latest in a string of leaks from Mr Snowden, who has told The Guardian he wants to expose "the largest programme of suspicionless surveillance in human history".
The newspaper said there were two principal components to the agency's surveillance programme, called Mastering the Internet and Global Telecoms Exploitation.
The paper claimed Operation Tempora had been running for 18 months.
GCHQ and the NSA can access communications including recordings of phone calls and a user's entire internet history, the documents suggest - and their scans apparently entirely innocent people as well as specific suspects.
Edward Snowden has been charged with espionageMr Snowden, who fled the US for Hong Kong after deciding to reveal the NSA's secrets, told the paper: "It's not just a US problem. The UK has a huge dog in this fight," he said.
"They (GCHQ) are worse than the US."
The Guardian reported that GCHQ lawyers told US counterparts there was a "light oversight regime" in Britain compared with America.
The newspaper said the documents revealed 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 intelligence gathered is understood to have led to a number of high-profile arrests and convictions, including a terror cell in the Midlands. It is also claimed to have led to the arrest of London-based individuals planning attacks prior to last year's Olympic Games.
A source close to the intelligence agencies told Sky News' Political Correspondent Sophy Ridge that GCHQ scanned data for possible indications of a threat to national security, and that most of the information is not looked at in detail.
The work is legal and subject to ministerial scrutiny, the source said.
Ridge said: "At the same time this is of course going to reignite the big debate over the balance between protecting national security and making sure that people's personal details aren't compromised."
Privacy campaign group Big Brother Watch's director Nick Pickes said: "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."
A GCHQ spokeswoman said: "Our intelligence agencies continue to adhere to a rigorous legal compliance regime."
US authorities have filed espionage charges against Mr Snowden, a former CIA technician who formerly worked for the NSA, and have asked Hong Kong to detain him.