David Cameron has been told that the government's attempt to destroy sensitive leaked documents about mass surveillance was "an act of intimidation" that risks a chilling effect on press freedom.

The World Association of Newspapers and News Publishers (WAN-IFRA) has written to the prime minister over the government's "deeply regrettable" response to files leaked by the National Security Agency whistleblower Edward Snowden.

The body, which represents 18,000 publications from 3,000 companies, condemned officials for their "symbolic" attempt to restrict reporting by destroying computer hard drives held by the Guardian.

"That your government felt the need to threaten legal action in order to block reporting into issues of public interest is deeply regrettable," the organisation wrote in its letter to the prime minister.

"Furthermore, WAN-IFRA is extremely concerned that the government's actions were an act of intimidation that could have a chilling effect on press freedom in the UK and beyond."

It described as "outrageous and deeply disturbing" the detention of David Miranda, the partner of Guardian journalist Glenn Greenwald, at Heathrow airport under Schedule 7 of the Terrorism Act 2000.

"The apparent misuse of this particular element of anti-terror legislation places journalists, and those aiding journalistic work, under suspicion of being terrorists or having involvement in terrorist activities," said WAN-IFRA.

"This is an outrageous and deeply disturbing connection to make, and we seek assurances from you and your government that the necessary inquiries will be made to ensure any inference of association between journalism and terrorism is not part of official policy and is publicly condemned as categorically misleading."

The letter to Cameron came as the Guardian shared some of its documents leaked by Snowden with the New York Times to circumvent what the newspaper described as "a climate of intense pressure" from the government. Journalists in America are protected by the first amendment which guarantees free speech and in practice prevents the state seeking pre-publication injunctions or prior restraint.

In the letter, signed by the WAN-IFRA president Tomas Brunegård and World Editors Forum president Erik Bjerager, the body added its concern over a "perceived slide in press freedom" in the UK, citing the arrest of dozens of journalists and questions over the future of press regulation following Lord Justice Leveson's report.

It added: "We respectfully call on you to reaffirm the United Kingdom's commitment to a free and independent press and to realign the various government and police authorities behind this unequivocal message.

"We urge the UK government to respect the rights of journalists to protect their sources and to create the conditions necessary to ensure the press can continue its crucial role in maintaining free and fair societies, without government interference or intimidation."

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