WILLIAM Hague will today try to reassure MPs over concerns the UK security services have used an American surveillance network to spy on British citizens after dismissing as "fanciful nonsense" the idea GCHQ, the UK Government's listening centre, has circumvented the law to do so.

The Foreign Secretary, who will make a Commons statement this afternoon, insisted law-abiding people had nothing to fear from the security services, which he insisted acted within the law and were subject to stringent scrutiny.

"The idea that in GCHQ people are sitting working out how to circumvent a UK law with another agency in another country is fanciful."

It is thought today GCHQ will provide Westminster's Intelligence and Security Committee with evidence on its monitoring activities.

Leaked documents allegedly show Prism, a surveillance system operated by America's National Security Agency (NSA), let GCHQ circumvent formal legal processes needed to obtain personal material such as emails, photographs and videos from internet firms based outside the UK.

Labour's Keith Vaz, chairman of the Commons Home Affairs Committee, said the allegations were chilling and, if true, appeared to be "the snooper's charter by the back door". Prism is said to give the 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 access to their servers.

Mr Hague declined to confirm he had authorised engagement with the Prism programme but he emphasised the strong legal and monitoring check British security services worked to.

"If you are a law-abiding citizen of this country going about your business and personal life you have nothing to fear about the British state or intelligence agencies listening to the content of your phone calls."

Major-General Jonathan Shaw, who until last year was head of cyber security at the Ministry of Defence, defended the British intelligence services. "What we're seeing is an incredibly difficult job of getting the balance right between security and freedom. We should recognise this link with America, between GCHQ and the NSA, is absolutely essential to both our countries' security."

l An American contract employee at the National Security Agency is the source of disclosures about the NSA's work. The leaks led the NSA to ask the US Justice Department for a criminal probe.

Edward Snowden, 29, an employee of defence contractor Booz Allen Hamilton and a former CIA worker, said: "I have no intention of hiding who I am because I know I have done nothing wrong."