David Cameron has said British intelligence agencies operate "within a legal framework", as MPs prepare to grill William Hague on GCHQ's involvement with the American Prism internet surveillance system.
"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."
The foreign secretary, who is due to make a statement on the allegations in the Commons later, has said the law-abiding British public had "nothing to fear" from the work of GCHQ.
However MPs are likely to press Hague on whether the intelligence service has always abided by the legal framework.
Sir Malcolm Rifkind, the chairman of the ISC, told BBC Radio 4's Today programme that GCHQ would have needed to ask ministers before requesting information on British citizens' internet activity from the United States.
"The law is actually quite clear: if the British intelligence agencies are seeking to know the content of emails by people living in the UK, then they actually have to get lawful authority. Normally that means ministerial authority," he said.
"That applies equally whether they are going to do the intercept themselves or whether they are going to ask somebody else to do it on their behalf."
The whistleblower who exposed the programme has condemned the US government's attempts "to destroy privacy, internet freedom and basic liberty".
Details about the programme, and GCHQ's links to it, emerged in The Guardian newspaper 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."
Prism is said to give the NSA and FBI easy access to the systems of nine of the world's top internet companies, including Google, Facebook, Microsoft, Apple, Yahoo and Skype.
The row crossed the Atlantic after documents emerged suggesting British eavesdropping agency GCHQ had access to the system since at least June 2010.
On Monday Downing Street said the UK has "exceptional" intelligence sharing with the US but said it would not comment on specific trans-atlantic conversations that may or may not have happened in relation to Prism.