British authorities forced The Guardian newspaper to destroy material leaked by whistle-blower Edward Snowden, its editor has revealed, calling it a "pointless" move that would not prevent further reporting on US and British surveillance programs.

In a column in the paper on Tuesday, Alan Rusbridger said the "bizarre" episode a month ago and the detention at London's Heathrow airport on Sunday of the partner of a Guardian journalist showed the press freedom was under threat in Britain.

London's Metropolitan Police defended the detention under an anti-terrorism law of David Miranda, the Brazilian partner of US journalist Glenn Greenwald, saying it was "legally and procedurally sound".

Miranda, a Brazilian who was in transit on his way from Berlin to Rio de Janeiro where he lives with Greenwald, was questioned for nine hours before being released without charge without his laptop, mobile phone and memory sticks.

"This law shouldn't be given to police officers. They use it to get access to documents or people that they cannot get the legal way through courts or judges. It's a total abuse of power," Miranda said.

Greenwald was the first journalist to publish US and British intelligence secrets leaked by Snowden, the former US National Security Agency contractor who is wanted in the United States and has found temporary asylum in Russia.

Greenwald, who has met Snowden and written or co-authored many Guardian stories about US surveillance of global communications, vowed to publish more revelations and said Britain would "regret" detaining his partner.

White House spokesman Josh Earnest told reporters on Monday that while Washington did not ask British authorities to detain Miranda, Britain had given the US a "heads up" about plans to question him.

A US security official told Reuters that one of the main purposes of Miranda's detention was to send a message to recipients of Snowden's materials that the British government was serious about shutting down the leaks.

Rusbridger said that a month ago, after The Guardian had published several stories based on Snowden's material, a British official advised him: "You've had your fun. Now we want the stuff back."

Rusbridger said the paper was threatened with legal action by the government unless it destroyed or handed over the material from Snowden.

After further talks with the government, two "security experts" from Government Communications Headquarters, the secretive British equivalent of the NSA, visited The Guardian's London offices.

In the building's basement, Rusbridger wrote, government officials watched as computers that contained material provided by Snowden were physically pulverized. "We can call off the black helicopters," one of the officials joked, Rusbridger said.

Reuters