NSA whistleblower Edward Snowden has said the UK's GCHQ (General Communications Headquarters) is at the forefront of surveillance techniques and is even more advanced than its counterparts in the US.
Snowden, who is on the run from US prosecutors after lifting the lid on the US data-mining scandal, told German magazine Der Spiegel that the UK was part of the so-called "Five Eyes" intelligence alliance with the US National Security Agency, Canada, Australia and New Zealand.
The GCHQ's Tempora programme mines every bit of computer data and misses nothing, Snowden told the Hamburg-based weekly.
"Tempora is the first 'save everything' approach ['full take'] in the intelligence world. It doesn't care about content type," Snowden said.
"It sucks in everything in a rolling buffer to allow retroactive investigation without missing a single bit."
Snowden said that GCHQ's data-snooping was so extensive that in an ideal world he would advise against digital date being routed through the UK "under any circumstances" - although that is something over which online users have little or no control.
"There is no way that you as an ordinary internet user can say I want my data to be routed this or that way," said Philipp Blank of Deutsche Telekom.
Klaus Landedfeld, a board member in charge of infrastructure and networks at the German Internet industry association Eco, added: "You've got no influence over that as the end-user."
Theoretically, he added, a user could try to influence the data flow by changing one's telecommunications provider - "not every undersea cable runs via Great Britain." But the providers constantly change the cables they send their customers' data through.
The 30-year-old former NSA contractor explained that unlike other surveillance systems, which collect only metadata, Tempora vacuums every bit of data.
"If you send a single ICMP [Internet Control Message Protocol] packet and it routes through the UK, we get it," Snowden said.
"If you download something and the CDN [Content Delivery Network] happens to serve from the UK, we get it. If your sick daughter's medical records get processed at a London call centre ... well, you get the idea."
Snowden, who is wanted on espionage charges, said Tempora allows GCHQ to save only three days of online traffic but there was "margin for improvement".
In the interview, which Der Spiegel said took place prior to Snowden fleeing his home in Hawaii for Hong Kong in May, the fugitive whistleblower also maintained that the US cooperated with Israel in the development of the Stuxnet computer virus used to target an Iranian nuclear facility.
Snowden, who remains stuck in legal limbo in the transit area of Moscow's Sheremetyevo airport.
He has filed for asylum with 27 countries but only Venezuela and Bolivia have said they are ready to receive him.