The British government is under increasing pressure to order an inquiry into claims that the country's security services were covertly gathering a mass of personal data from some of the world's largest Internet firms through a US spy programme.
Britain's eavesdropping centre Government Communications Headquarters will report to the Parliament's Intelligence and Security Committee Committee by Monday over claims it secretly gathered intelligence.
British Indian Member of Parliament Keith Vaz described the allegations regarding the government's Cheltenham-based electronic listening post as "chilling".
"The most chilling aspect is that ordinary American citizens and potentially British citizens too were apparently unaware that their phone and online interactions could be watched. I am astonished by these revelations, which could involve the data of thousands of Britons.
"This seems be the snoopers charter by the back door," said the Labour MP and chairman of the House of Commons' influential Home Affairs Select Committee. ISC chairman Sir Malcolm Rifkind added that the parliamentary committee would be "receiving a full report from GCHQ very shortly and will decide what further action needs to be taken as soon as it receives that information".
According to the report in the 'Guardian' newspaper, the GCHQ may have directly requested material from the US National Security Agency's Prism programme, which can search huge stores of internet data for information on individuals.
Prism was created by US intelligence to gather information on non-US citizens living outside the country. The British prime minister is now facing calls to order an investigation and MPs are demanding that either foreign secretary William Hague, who oversees the work of GCHQ, or the home secretary, Theresa May, appear in the Commons on Monday to explain how much ministers knew about the operation and how many British citizens could be affected.
US-run Prism would appear to have allowed GCHQ to circumvent the formal legal process required to seek personal material such as emails, photos and videos from an internet company based outside the United Kingdom.
According to documents obtained by the 'Guardian', the agency generated 197 intelligence reports from Prism last year, and has had access to the programme since at least 2010.
Asked to comment on its use of Prism, the GCHQ said it "takes its obligations under the law very seriously."
"Our work is carried out in accordance with a strict legal and policy framework which ensures that our activities are authorised, necessary and proportionate, and that there is rigorous oversight, including from the secretary of state, the interception and intelligence services commissioners and the intelligence and security committee".