BRITISH spies are running an online eavesdropping operation so vast that internal documents say it even outstrips the United States' international internet surveillance effort, The Guardian newspaper says.

The paper cited UK intelligence memos leaked by former National Security Agency contractor Edward Snowden to claim that UK spies were tapping into the world's network of fibre optic cables to deliver the "biggest internet access" of any member of the Five Eyes - the name given to the espionage alliance composed of the United States, Britain, Canada, Australia, and New Zealand.

That access could in theory expose a huge chunk of the world's everyday communications - including the content of people's emails, calls, and more - to scrutiny from British spies and their US allies. How much data the Brits are copying off the fibre optic network isn't clear, but it's likely to be enormous.

The Guardian said the information flowing across more than 200 cables was being monitored by more than 500 analysts from the NSA and its UK counterpart, GCHQ.

"This is a massive amount of data!" The Guardian quoted a leaked slide as boasting.

The newspaper, whose revelations about America and Britain's globe-spanning surveillance programs have reignited an international debate over the ethics of espionage, said GCHQ was using probes to capture and copy data as it crisscrossed the Atlantic between western Europe and North America.

It said that, by last year, GCHQ was in some way handling 600 million telecommunications every day - although it did not go into any further detail and it was not clear whether that meant that GCHQ could systematically record or even track all the electronic movement at once.

GCHQ declined to comment on Friday, although in an emailed statement it repeated past assurances about the legality of its actions.

"Our work is carried out in accordance with a strict legal and policy framework which ensures that our activities are authorised, necessary, and proportionate," the statement said.

Fibre optic cables - thin strands of glass bundled together and strung out underground or across the oceans - play a critical role in keeping the world connected. A 2010 estimate suggested that such cables are responsible for 95 per cent of the world's international voice and data traffic, and The Guardian said Britain's geographic position on Europe's western fringe gave it natural access to many of the trans-Atlantic cables as they emerged from the sea.

The Guardian said GCHQ's probes did more than just monitor the data live; British eavesdroppers can store content for three days and metadata - information about who was talking to whom, for how long, from where, and through what medium - for 30 days.