A British intelligence agency, GCHQ, has tapped into undersea fibre-optic cables to hoover up telephone conversations and Internet traffic, according to documents seen by The Guardian newspaper.

Codenamed "Tempora," the secret surveillance programme is said to be on an even larger scale than the US-led Prism scheme revealed by US whistleblower Edward Snowden earlier this month.

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“They [GCHQ] are worse than the US,” Snowden, a former analyst at the US' National Security Agency (NSA), told The Guardian on Friday (21 June).

Tempora is said to scoop up as much traffic as possible and to store it for analysis over 30-day periods.

Each day of the electronic dragnet is the equivalent of some 20 petabytes of data. One petabyte of high definition films would take 13 years to watch nonstop.

The Guardian says that GCHQ scoured through 600 million “telephone events” each day and tapped into more than 200 fibre-optic cables.

It was able to harvest data from 46 of the transatlantic cables.

The Tempora programme has been in operation for the past 18 months.

The latest revelation has sparked anger in Germany.

On Saturday, Germany’s justice minister Sabine Leutheusser-Schnarrenberger described Tempora as a “catastrophe” if true, Reuters reports.

“The accusations against Great Britain sound like a Hollywood nightmare. The European institutions should seek straight away to clarify the situation,” she said.

Germany’s opposition Social Democrat Thomas Oppermann described the scenario as "Orwellian" in the Frankfurter Allgemeine Sonntagszeitung, referring to George Orwell, the British writer of dystopian novel 1984.

For his part, William Hague, the UK’s foreign minister, earlier this month defended GCHQ surveillance programmes in the House of Commons.

He said every request to intercept the content of an individual’s communication “requires a warrant signed personally by me” or another secretary of state."

“This is no casual process,” he said.

But The Guardian on Friday portrayed Britain’s oversight regime as “light” when compared to the American equivalent.

It said UK officials boasted about their unprecedented access and ability to collect more data than the US, UK, Canada, Australia and New Zealand.

Meanwhile, Snowden has requested asylum in Ecuador after the US officially charged him with espionage, theft of government property, and unauthorised communication of national defence information.

The former NSA analyst was in Hong Kong when The Guardian and Washington Post started to issue a series of articles based on his leaked documents.

Snowden is reportedly now in Moscow.