William Hague: UK agencies uphold the law at all times
Allegations that the data-gathering centre GCHQ circumvented the law to gain information on UK citizens are "baseless", William Hague has said.
Ex-CIA worker Edward Snowden claims US agencies gathered and shared phone records and internet data with allies.
But the foreign secretary said people could have confidence in the work of UK security agencies and "their adherence to the law and democratic values".
GCHQ is to hand a report to MPs on Tuesday, the government has confirmed.
Mr Hague refused to confirm or deny claims GCHQ has had access to a US spy programme called Prism since June 2010.
It is said to give America's National Security Agency (NSA) and the FBI easy access to the systems of nine of the world's top internet companies, including Google, Facebook, Microsoft, Apple, Yahoo and Skype.
All deny giving the US government direct access to their servers.
In a statement to Parliament, Mr Hague said the UK's information-sharing relationship with the US was "essential to the security of the country" and had "saved many lives".
"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.
"I wish to be absolutely clear that this accusation is baseless."
Data obtained by the UK from the United States involving UK nationals, he added, was "subject to proper UK statutory controls and safeguards".
"Our intelligence-sharing work with the United States is subject to ministerial and independent oversight and to scrutiny by the Intelligence and Security Committee. Our agencies practise and uphold UK law at all times, even when dealing with information from outside the United Kingdom."
Speaking earlier at an event in Essex, Prime Minister David Cameron said the UK's intelligence agencies were subject to proper scrutiny by the Intelligence and Security Committee in the House of Commons.
"That scrutiny is important and I will make sure that it takes place."
He added: "I'm satisfied that we have intelligence agencies that do a fantastic job to keep us safe and operate within the law."
The chairman of the Intelligence and Security Committee, Conservative MP Sir Malcolm Rifkind, told BBC Radio 4's Today programme the issues concerned were complex but "the law is quite clear".
"If the British intelligence agencies are seeking to know the content of emails about people living in the United Kingdom, then they have to get authority," he said. "That means ministerial authority."
Former CIA technical worker Edward Snowden has been identified by the Guardian newspaper as the source of leaks about US surveillance programmes.
GCHQ, based in Cheltenham, is said to have generated 197 intelligence reports through the system in the 12 months to May 2012 - a 137% increase on the previous year.
'In the dark'
For Labour, shadow foreign secretary Douglas Alexander told MPs the British public needed to be assured that "the agencies of government are law-abiding".
He sought guarantees that data supplied by the US could only be accessed with the personal authorisation of the foreign secretary, or an equivalent cabinet minister, and with the oversight of relevant watchdogs.
And Conservative MP Dominic Raab said it was not Parliament's job to "blindly trust" the intelligence agencies and he was concerned that MPs and the interceptions of communications commissioner had been "kept in the dark".
Without a "watertight" legal basis for surveillance, public confidence would be eroded and the UK risked being sued by victims, he said.
The BBC's security correspondent Frank Gardner said ministers were being "very cautious" in their comments about intelligence sharing so as not to give any advantage to terrorist suspects.
But while they had made the legal position clear, he said their responses did not address questions about the NSA's activities and what its "enormous data storage capacity" was being used for.
Prism was reportedly established in 2007 in order to provide in-depth surveillance on live communications and stored information on foreigners overseas.
The content of phone conversations - what people say to each other when they are on the phone - is protected by the Fourth Amendment to the US Constitution, which forbids unreasonable searches.
However, information shared with a third party, such as phone companies, is not out of bounds.
That means that data about phone calls - such as their timing and duration - can be scooped up by government officials.
Prism was authorised under changes to US surveillance laws passed under President George W Bush and renewed last year under Barack Obama. He has defended the surveillance programmes as a "modest encroachment" on privacy, needed to protect the US from terrorist attacks.