The Government Communications Headquarters, the British equivalent of the U.S. National Security Agency, conducted the monitoring under a program codenamed Squeaky Dolphin, NBC News has reported.
GCHQ demonstrated its monitoring abilities to the NSA in 2012, NBC said.
Both agencies apparently have been piggybacking off customer data that software companies collect themselves, without those companies' knowledge.
Since the revelations Facebook has begun encrypting its data, but Google's YouTube and Blogger services remain unencrypted, although the company has begun taking measures to protect users' data.
"We have long been concerned about the possibility of this kind of snooping, which is why we have continued to extend encryption across more and more Google services and links," a Google spokesman told the BBC.
"We do not provide any government, including the U.K. government, with access to our systems," he said. "These allegations underscore the urgent need for reform of government surveillance practices."
A GCHQ spokeswoman responded, saying the agency's surveillance activities are carried out in accordance with British laws.
"All of GCHQ's work is carried out in accordance with a strict legal and policy framework which ensures that our activities are authorized, necessary and proportionate, and that there is rigorous oversight, including from the secretary of state, the Interception and Intelligence Services Commissioners and the Parliamentary Intelligence and Security Committee," she said.