Ministers should examine whether the United States has used its Prism system to spy on British companies as well as the UK government, a Lib Dem member of the Commons home affairs committee has said.
Last week The Guardian revealed the existence of a surveillance programme that is said to give America's National Security Agency and FBI easy access to the systems of nine of the world's top internet companies, including Google, Facebook, Microsoft, Apple, Yahoo and Skype.
Lib Dem MP Julian Huppert said this raised serious questions about cyber security. "A lot of people, a lot of companies use Gmail," he said. The Google email system is also used within Whitehall.
"If the US government can get access, that has huge implications for everybody," Huppert told HuffPost UK. "Are we happy with the idea the US can get acces to information about what a British company, which might be competing with a US company, is doing on its Microsoft exchange server?"
David Cameron has said British intelligence agencies operate "within a legal framework".
"I think it is right that we have well-organised, well-funded intelligence services to help keep us safe," the prime minister said on Monday morning.
"But let me be absolutely clear. They are intelligence services that operate within the law, within a law that we have laid down, and they are also subject to proper scrutiny by the intelligence and security committee (ISC) in the House of Commons."
William Hague, who is due to make a statement on the allegations in the Commons on Monday afternoon, has said the law-abiding British public had "nothing to fear" from the work of GCHQ.
Huppert said the foreign secretary would need to reassure MPs that "at no stage" did GCHQ break the law. "We need to know whether the UK knew about this at a ministerial level," he said.
The Lib Dem backbencher also said the revelations about American internet surveillance emphasises why the Conservative supported so-called 'snoopers charter' which would give the British government greater powers to look at internet traffic was a "bad idea".
"People don’t think it is right for the government to have this broad access," he said. "The public are now far more aware of this."
Campaigners warned last week that the snooper's charter would open a "Pandora's box that could turn us into suspects, not citizens," in light of the NSA Prism scandal.
Details about the programme, and GCHQ's links to it, emerged in The Guardian following a leak by a former technical worker at the CIA and National Security Agency (NSA).
Edward Snowden, 29, an American IT administrator for the defence contractor Booz Allen Hamilton, revealed his identity at his own request, the newspaper said.
Snowden, who fled to Hong Kong, said: "I can't allow the US government to destroy privacy, internet freedom and basic liberties. My sole motive is to inform the public as to that which is done in their name and that which is done against them."