Documents show GCHQ (above) has had access to the NSA's Prism programme since at least June 2010. Photograph: David Goddard/Getty Images

The UK's electronic eavesdropping and security agency, GCHQ, has been secretly gathering intelligence from the world's biggest internet companies through a covertly run operation set up by America's top spy agency, documents obtained by the Guardian reveal.

In its statement about Prism, Google said it "cares deeply about the security of our users' data. We disclose user data to government in accordance with the law, and we review all such requests carefully. From time to time, people allege that we have created a government 'back door' into our systems, but Google does not have a back door for the government to access private user data".

In a statement confirming the existence of Prism, James Clapper, the director of national intelligence in the US, said: "Information collected under this programme is among the most important and valuable intelligence information we collect, and is used to protect our nation from a wide variety of threats."