Media playback is unsupported on your device Media caption Gordon Corera reports: "Tapping into the huge flows of internet data has become a top priority"

Google has issued a strong denial that it allows the US government to access its servers.

The internet giant said the government has no access, "not directly, or via a back door, or a so-called drop box".

The Guardian claims the UK's eavesdropping centre GCHQ has secretly gathered intelligence on Britons from the world's largest internet companies.

GCHQ is to report to MPs within days over claims it accessed data through a US spy programme called Prism.

Parliament's Intelligence and Security Committee (ISC) expects the report by Monday.

GCHQ said in a statement it operated to "a strict legal and policy framework".

US spies have been accused of tapping into servers of nine US internet giants including Apple, Facebook, Microsoft and Google in a giant anti-terror sweep. All deny giving government agents access to servers.

Google's Chief Legal Officer David Drummond said: "We cannot say this more clearly - the government does not have access to Google servers...It is quite wrong to insinuate otherwise.

What we'd like to know is: has this actually had the authority of ministers and how long has it been going on for? This is not the traditional route of spookology... Keith Vaz, Home Affairs Select Committee chairman

"We provide user data to governments only in accordance with the law. Our legal team reviews each and every request, and frequently pushes back when requests are overly broad or don't follow the correct process."

The Guardian said it has obtained documents showing that Britain's secret listening post had access to the Prism system, set up by America's National Security Agency (NSA), since at least June 2010.

The newspaper said the Prism programme appeared to allow the Government Communications Headquarters (GCHQ) to circumvent the formal legal process required to obtain personal material, such as emails, photographs and videos, from internet companies based outside the UK.

Microsoft founder Bill Gates, in London for a Hyde Park rally calling for action to end hunger, said he knew nothing about Prism, adding: "I don't know any specifics but if there's a court order for companies to do things it's typical that they're obeyed."

Washington visit

ISC chairman Sir Malcolm Rifkind said 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".

Media playback is unsupported on your device Media caption Douglas Alexander: "These latest reports over the last 48 hours have raised very real public concerns"

Committee members will discuss the claims with US security officials during a planned visit to Washington next week.

Shadow foreign secretary Douglas Alexander has urged the foreign secretary - the minister with responsibility for GCHQ - to make a statement to Parliament on the reports.

He told the BBC: "I am calling on William Hague, as the foreign secretary, to come to the dispatch box of the House of Commons on Monday to set out the government's position and explain how the government will work with the ISC to address the very real public concerns that have emerged."

Labour's Keith Vaz, chairman of the Home Affairs Select Committee, said that while the security services need to share information with the UK's closest ally, the US, certain questions need to be answered.

He told the BBC: "I think what we'd like to know is, has this actually had the authority of ministers and how long has it been going on for?

"This is not the traditional route of spookology because normally you would go and get an order and that order would be subject to proper accountability and judicial process. This is something that is, in a sense, possibly through the back door."

BBC security correspondent Gordon Corera said intelligence and law enforcement agencies in the UK already had the ability to request information from internet and other companies through court orders and warrants.

"The question about Prism is whether it is simply an interface with the companies to then get hold of that information, or a kind of dragnet to gather vast amounts of information about everyone to sift through," our correspondent said.

He added: "Even more worryingly, was it a means of evading the legal oversight and legal restriction on how they operated?"

Shami Chakrabarti, director of human rights group Liberty, said ministers were "going to have to say whether they knew about this and what they have and haven't authorised".

Media playback is unsupported on your device Media caption Shami Chakrabarti, Liberty: "[Spies] have to be accountable to Parliament and the law"

She told the BBC Radio 4's Today programme the big question was "have our agencies been circumventing the law by this nice little international exchange"?

She added: "The danger with this and other security policies is that governments say, 'well we'll obey our own laws that protect our citizens but we will then play fast and loose with the freedoms of others on the other side of the Atlantic or wherever'."

Former Labour MP Kim Howells, who chaired the ISC from 2008-2010, told Today he did not believe "for one minute" that intelligence service chiefs would be prepared to "venture into areas that are clearly illegal without, if you like, the permission of the government".

'Deeply concerning'

Conservative MP David Davis said the claims indicated the intelligence services had "much more information than they used to have… namely our most intimate traffic".

That could include "a love email sent to your wife or mistress or girlfriend", he added.

Privacy is effectively a 20th century concept like the steam engine Richard Aldrich, International security professor

"That sort of thing's now available through a mechanism which doesn't go through the British courts and that's pretty serious."

US President Barack Obama, meanwhile, has defended the Prism monitoring programme, saying it was closely overseen by Congress and the courts and that his administration had struck "the right balance" between security and privacy.

Richard Aldrich, a professor of international security at the University of Warwick, said he expected Mr Cameron to say "rather as President Obama has said, that you can't have your cake and eat it - you can't have 100% privacy and 100% security".

"What they're not going to say is, actually, we're very rapidly accelerating to a point where we're going to be in a transparent society," he told Today.

"Privacy is effectively a 20th century concept like the steam engine."

A spokesman for the agency, based in Cheltenham, said: "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."

Meanwhile, the BBC has learned that Twitter was invited to join the Prism programme last year, but rejected the approach from US authorities.