A group of the world's leading press freedom bodies is calling on prime minister David Cameron to distance himself from the investigation into The Guardian over the leaks by the NSA whistleblower Edward Snowden.

The seven organisations also want Cameron to urge parliament to repeal the statute that underlies the royal charter on press regulation.

Signatories to a letter sent to Cameron today include the World Association of Newspaper and News Publishers (WAN-IFRA), the New York-based Committee to Protect Journalists (CPJ) and the International Press Institute (IPI).

The decision to write to Cameron was taken at the annual meeting of the global coordinating committee of press freedom organisations, which took place in London last month. It followed what the signatories call an "unprecedented" fact-finding mission to Britain by WAN-IFRA.

It points out that "Britain's democracy, including its robust and diverse media, has been an inspiration to people around the world who struggle to be free, and is a source of British 'soft power' and influence". It continues:

"Any action that diminishes that perception not only emboldens autocratic leaders to take repressive action against the media but it erodes the ability of Britain to exercise moral suasion and to defend the rights of the world's most vulnerable journalists."

A lengthy section is devoted to "the government pressure" on The Guardian and its editor, Alan Rusbridger, from May 2013 onwards following the publication of articles based on documents leaked by Snowden, which revealed mass surveillance carried out by the NSA and GCHQ.

The letter notes that the story sparked an international debate and "a broad policy review in the United States that is playing out both in the courts and the political arena." But, it continues:

"Unfortunately, the focus of attention in the UK has been less on the implications of the Snowden revelations and more on the journalistic efforts undertaken by The Guardian to make this critically important information available to the global public."

It refers to the detention at Heathrow last August of David Miranda, the partner of the then Guardian reporter Glenn Greenwald, under anti-terrorism legislation, and the subsequent requirement on The Guardian to destroy hard drives containing the leaked documents.

It mentions the fact that Rusbridger was called before a parliamentary committee in December during which his "patriotism was called into question." Then the letter addresses Cameron personally:

"Speaking before parliament, you claimed, without evidence, that The Guardian's actions had damaged British national security and urged parliament to carry out an investigation. Parallel to the parliamentary investigation, the Metropolitan Police are reportedly carrying out a criminal inquiry into possible violations of the anti-terror law. We view these actions and the consistent government pressure on The Guardian as incompatible with the British tradition of press freedom, and deeply damaging to the country's international prestige. If there is evidence that The Guardian has broken the law – and we would like to stress that we have seen absolutely nothing to suggest that this is the case — then the competent judicial authorities should carry out an independent criminal investigation free of government interference. Your comments, and those of some members of parliament, have at a minimum undermined the perception of impartiality by suggesting that the process is being driven by political rather than legal concerns."

The letter then turns to the "fierce debate over media regulation" and argues that the two issues are linked because "together they create the impression that British authorities are seeking to constrain and control the work of the media."

It calls parliament's decision to use the "medieval vestige" of a royal charter to under pin a new form of press regulation "a convoluted process virtually incomprehensible to anyone not versed in its arcana." It states:

"This parliamentary action, in our view, establishes statutory under pinning for media regulation. This means that the Rubicon has, in fact, been crossed. After listening to all sides of the debate, we recognise the gravity of the problem of media abuse that the royal charter seeks to address... It is our view that the parliamentary action that essentially compels participation in the regulatory mechanism belies claims that it will be 'voluntary.' Indeed, it should be a source of serious concern to your government that autocratic leaders seeking to limit media freedom now cite the British example."

The letter adds: " We write to urge you to take immediate steps to safeguard press freedom in the UK and to ensure that the actions of your government are never used to justify media restrictions elsewhere in the world."

Source: CPJ Full letter: Pdf document