Image caption The Guardian said the operation has been running for 18 months

The UK is tapping fibre-optic cables that carry global communications and gathering vast amounts of data, the Guardian has reported.

The newspaper said its report was based on leaked documents from GCHQ, the UK's electronic eavesdropping agency.

The information from internet and phone use was stored for up to 30 days to be sifted and analysed, the paper said.

GCHQ declined to comment on the claims but said its compliance with the law was "scrupulous".

The documents were reportedly released by Edward Snowden, the former IT contractor at GCHQ's US counterpart, the National Security Agency, who is believed to be in hiding in Hong Kong.

Mr Snowden is behind a string of other disclosures about US intelligence operations. It is reported that US authorities have charged him with espionage and theft.

According to the Guardian, the GCHQ operation codenamed Tempora has been running for 18 months.

The paper said GCHQ was able to boast a larger collection of data than the US, tapping in to 200 fibre-optic cables to give it the ability to monitor up to 600 million communications every day.

Media playback is unsupported on your device Media caption Sir Malcolm Rifkind and Shami Chakrabarti on GCHQ information

BBC security correspondent Gordon Corera says the Guardian is not accusing GCHQ of breaking the law but it does suggest the existing legislation is being very broadly applied to allow such a large volume of data to be collected.

In 2009, GCHQ issued an unusual statement in which it said it had no ambitions, expectations or plans for a database to store centrally all communications data in Britain, our correspondent added.

Nick Pickles, from privacy campaign group Big Brother Watch, said: "This appears to be dangerously close to, if not exactly, the centralised database of all our internet communications, including some content, that successive governments have ruled out and Parliament has never legislated for.

"If GCHQ have been intercepting huge numbers of innocent people's communications as part of a massive sweeping exercise, then I struggle to see how that squares with a process that requires a warrant for each individual intercept."

Shami Chakrabarti, director of human rights group Liberty, said she was shocked by the Guardian's report and accused GCHQ of allowing itself a "very generous interpretation of the law".

"They are exploiting the fact that the internet is so international in nature," she told BBC Radio 4's Today programme.

"And I'm pretty sad in a democracy when all that appears to be holding back the secret state is its physical and technological capability and not its ethics or a tight interpretation and application of the law."

In response to the Guardian report, a GCHQ spokeswoman said: "We do not comment on intelligence matters. It is worth pointing out that GCHQ 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."

Sir Malcolm Rifkind, chairman of the Intelligence and Security Committee which oversees the work of the UK's intelligence services, said he expected to see a written report from GCHQ in "the next day or so" about their response to the report.