The Guardian, a major outlet for revelations based on leaks from former US intelligence contractor Edward Snowden, says the British government threatened legal action against the newspaper unless it either destroyed the classified documents or handed them back to British authorities.

In an article posted on the British newspaper's website on Monday, Guardian editor Alan Rusbridger said that a month ago, after the newspaper 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's disclosure follows Sunday's detainment at London's Heathrow Airport under British anti-terrorism laws of David Miranda, domestic partner of US journalist and Guardian writer Glenn Greenwald.

Miranda, a Brazilian citizen in transit from Berlin to Brazil, said he was released without charge after nine hours of questioning but minus his laptop, cellphone and memory sticks.

Greenwald, who met face to face in Hong Kong with Snowden and has written or co-authored many of the newspaper's stories about US surveillance of global communications, vowed on Monday to publish more documents and said Britain will "regret" detaining his partner.

Rusbridger said that after further talks with the British government, two "security experts" from Government Communications Headquarters, the British equivalent of the ultra-secretive US National Security Agency, visited the Guardian's London offices.

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

A source familiar with the event said Guardian employees destroyed the computers as government security experts looked on.

Rusbridger, in his article, said he told British officials that due to the nature of "international collaborations" among journalists, it would remain possible for media organisations to "take advantage of the most permissive legal environments." Henceforth, he said, the Guardian "did not have to do our reporting from London."

A source familiar with the matter said that this meant British authorities were on notice that the Guardian was likely to continue to report on the Snowden revelations from outside British government jurisdiction.

'HAD YOUR DEBATE'

Rusbridger said that in meetings with British officials, before the computers were destroyed, he told them the Guardian could not do its journalistic duty if it gave in to the government's requests.

In response, he wrote, a government official told him that the newspaper had already achieved the aim of sparking a debate on government surveillance. "You've had your debate. There's no need to write any more," the unnamed official was quoted as saying.

The Guardian's decision to publicise the government threat - and the newspaper's assertion that it can continue reporting on the Snowden revelations from outside of Britain - appears to be the latest step in an escalating battle between the news media and governments over reporting of secret surveillance programmes.

One US security official told Reuters that one of the main purposes of the British government's detention and questioning of Miranda was to send a message to recipients of Snowden's materials, including the Guardian, that the British government was serious about trying to shut down the leaks.

White House spokesman Josh Earnest told reporters on Monday that while the United States did not ask British authorities to detain Miranda, British officials had given the US a "heads up" about the British government's plan to question him.

Greenwald, asked by a reporter if the detention of his partner would deter him from future reporting, said the opposite would happen.

"I will be far more aggressive in my reporting from now. I am going to publish many more documents. I am going to publish things on England, too. I have many documents on England's spy system," Greenwald, speaking in Portuguese, told reporters at Rio de Janeiro's airport where he met Miranda upon his return to Brazil.

During Miranda's trip to Berlin, which the Guardian said it had paid for, he visited with Laura Poitras, an independent film-maker who was the first journalist to interact with Snowden. Poitras co-authored stories based on Snowden's material for the Washington Post and the German magazine Der Spiegel.

Greenwald told the New York Times that Miranda went to Berlin to deliver materials downloaded by Snowden to Poitras and to acquire from Poitras a different set of materials for delivery to Greenwald, who lives with Miranda near Rio de Janeiro.

Greenwald told the Forbes website that "everything" Miranda had "was heavily encrypted." Greenwald did not immediately respond to an email from Reuters requesting comment.

While British authorities confirmed that Miranda had been detained under an anti-terrorism law, they did not further explain their actions. Brazil's government complained about Miranda's detention in a statement on Sunday that said the use of the British anti-terrorism law was unjustified. - Reuters