Britain’s Foreign Secretary William Hague has insisted that British spies have not used US surveillance programmes to get around UK privacy laws.
Hague, who cancelled a trip to Washington to address parliament on the issue on Monday, said Britain's electronic eavesdropping agency GCHQ operated within a strict legal framework.
"It has been suggested that GCHQ uses our partnership with the United States to get around UK law, obtaining information that they cannot legally obtain in the United Kingdom," Hague said.
"I wish to be absolutely clear that this accusation is baseless."
The Guardian newspaper along with The Washington Post revealed last week that the US is using a surveillance system code-named Prism, which allows the US National Security Agency (NSA) and the FBI direct access to the servers of US internet firms such as Google, Apple, Microsoft, Facebook and AOL.
Edward Snowden, a 29-year-old former CIA worker, revealed himself as the source of the leaked information on the secret spy programme.
The Guardian reported on Monday that GHCQ has had access to Prism since at least June 2010.
"We take great care to balance individual privacy with our duty to safeguard the public and the UK's national security.
Seeking to calm public concern, Hague told legislators that every time GCHQ wants to intercept an individual's communications the agency must seek a warrant signed by him, the interior minister or another secretary of state.
"Every decision is based on extensive legal and policy advice. Warrants are legally required to be necessary, proportionate and carefully targeted," he said.
He added: "We take great care to balance individual privacy with our duty to safeguard the public and the UK's national security."
Hague said that since the 1940s, the NSA and its forerunners have had a relationship with GCHQ that is "unique in the world".
This relationship "has stopped many terrorist and espionage plots against this country and it has saved many lives," he said.
But asked by an opposition spokesman if it was possible that GCHQ has made mistakes and spied on innocent members of the public, Hague admitted: "Everyone is capable of error... there will always be ways of improving our procedures."
Parliament's Intelligence and Security Committee scrutiny panel, which is investigating possible use of the US schemes by GCHQ, was on Monday travelling to Washington on a long-planned trip.
Hague deplored the leak of the surveillance programmes by a former CIA employee, saying they had made the work of protecting national security in the United States and beyond more difficult.
Snowden, while he was a government contractor at the NSA, said his conscience drove him to reveal the scale of the monitoring of internet users.