The detention and subsequent criminal investigation into the partner of a Guardian journalist threatens to undermine the position of the free press around the world, the editors of several northern European newspapers have warned.

In an open letter to David Cameron published in today's Observer, the editors of Denmark's Politiken, Sweden's Dagens Nyheter, Norway's Aftenposten and Finland's Helsingin Sanomat describe the detention of David Miranda, the partner of the Guardian's Glenn Greenwald, as harassment.

They say that the "events in Great Britain over the past week give rise to deep concern" and call on the British prime minister to "reinstall your government among the leading defenders of the free press".

Miranda was detained by the Metropolitan police for nine hours last Sunday as he was passing through Heathrow on his way to Brazil.

Greenwald has broken a series of stories about the US intelligence agencies based on material leaked by the National Security Agency whistleblower Edward Snowden.

The editors describe a free press as crucial to holding governments and their intelligence agencies to account. They write: "We are surprised by the recent acts by officials of your government against our colleagues at the Guardian and deeply concerned that a stout defender of democracy and free debate like the United Kingdom uses anti-terror legislation in order to legalise what amounts to harassment of both the paper and individuals associated with it."

They add: "It is deeply disturbing that the police have now announced a criminal investigation" and they warn that "the implication of these acts may have ramifications far beyond the borders of the UK, undermining the position of the free press throughout the world".

The letter's publication comes as it emerged that Scotland Yard will face legal action over its use of anti-terrorism powers to question people at airports unless it hands over the results of investigations into alleged misuse by its officers.

The Independent Police Complaints Commission (IPCC) has said it has given the force a seven-day ultimatum to reveal its findings into outstanding complaints about the use of the tactic following its "continued refusal" to investigate.

The watchdog said it was supervising 18 investigations into the use by the Met of Schedule 7 powers, which allow officers to detain passengers for up to nine hours without needing reasonable suspicion of involvement in terrorism.

The IPCC said it ordered the Met in February to "investigate the rationale for stopping and questioning people under Schedule 7". The force agreed to investigate two months later, following the threat of legal action, but then refused to hand over the resulting investigation documents to the watchdog, an IPCC spokesman said.

Scotland Yard said it was "working hard" to agree a procedure for dealing with investigations with the IPCC and and that legal action had so far been "unnecessary".

A Met spokesman said: "The Metropolitan Police Service recognises the IPCC's role in scrutinising complaints related to Schedule 7 stops and has been working hard to agree a procedure for dealing with such investigations that is acceptable to all stakeholders. As a result of these efforts, legal action has been unnecessary. We hope to be in a position to finalise a way forward with the IPCC in the future."

Meanwhile it has emerged that the US government's efforts to determine which highly classified materials Snowden took from the NSA have been frustrated by the former contractor's sophisticated efforts to cover his digital trail.

The Associated Press reported that the US government investigation is examining whether Snowden was able to defeat safeguards established to deter people looking at information without proper permission by deleting or bypassing electronic logs.

In July, nearly two months after Snowden's earliest disclosures, the NSA director, Keith Alexander, declined to say whether he had established what Snowden had downloaded or how many NSA files he had taken with him.

The latest disclosure undermines the Obama administration's assurances to Congress and the public that the NSA surveillance programs cannot be abused because its spying systems are so aggressively monitored and audited.

Fears about government snooping are now a major concern for internet companies, which are examining measures to restrict external surveillance of people's online activity.

The Internet Engineering Task Force, a body that develops internet standards, has proposed a system in which communication between websites and browers would be shielded by encryption. The proposals, which are at an early stage, would make it harder for governments, companies and criminals to eavesdrop on people as they browse the web.