The Government Communications HQ (GCHQ) in Britain worked with US intelligent agency NSA to collect data from Yahoo and Google, documents leaked by whistle-blower Edward Snowden suggests.
It has been suggested that up to 181,280,466 records a month were sent back to the NSA headquarters, including people who sent and received emails.
Prime Minister David Cameron has defended the agencies, and has said that the leaks from Snowden were making it more difficult to make people safe.
Google said that it was "troubled by allegations of the government intercepting traffic between our data centres”, adding that it had not been aware of the activity.
"We have long been concerned about the possibility of this kind of snooping, which is why we continue to extend encryption across more and more Google services and links," it added.
Previous Snowden leaks revealed that communications data was sought from the internet giants, but the new leaks suggests that "entire data flows" have been copied through the fibre-optic cables carrying information between the two firms' data centres.