A CIVIL liberties group has heaped further pressure on the Government to come clean over claims that GCHQ is monitoring vast quantities of private information.

Liberty director Shami Chakrabarti yesterday called on Prime Minister David Cameron to tell MPs how the alleged data harvesting could be justified.

Ms Chakrabarti said: "Revelations of blanket surveillance of the British public on such a scale amount to a huge scandal even by the standards of recent years.

"At the very least, the Prime Minister or Foreign Secretary should appear before the House of Commons immediately to explain how this was justified without clear legal authority or parliamentary debate."

Claims about GCHQs secret access to information carried on high speed fibre optic cables follow a leak from US National Security Agency contractor Edward Snowden, who is now on the run.

It has been reported GCHQ, the UK's eavesdropping agency, and the NSA have been monitoring vast quantities of communications between innocent people as well as targeted suspects, including phone calls, the content of email messages, Facebook entries and a user's internet history, as part of an 18-month long operation known as Tempora.

Kenny MacAskill, the Scottish Justice Secretary, wrote to Foreign Secretary William Hague at the weekend seeking assurances that Scots law had not been breached.

Meanwhile, Sir Malcolm Rifkind, the Tory chairman of the Intelligence and Security Committee, said he wants a written report from GCHQ about the claims.

Former Foreign Office minister David Davis said reports GCHQ lawyers told US counterparts there was a "light oversight regime" in Britain compared with America were "worrying".

A GCHQ spokeswoman said: "We do not comment on intelligence matters. Our intelligence agencies continue to adhere to a rigorous legal compliance regime. GCHQ are scrupulous in their legal compliance."

Earlier this month MSP Margo MacDonald sought assurance from the UK's Security Service that MI5 spies would not interfere in the independence referendum.