The NUJ is considering legal action on behalf of a group of Sun journalists over News Corp's decision to hand over details of confidential sources to the Met.
One avenue being explored by the union is taking action citing Article 10 of the Human Rights Act (freedom of expression), which protects journalists and their sources under a 2007 ruling at the European Court of Human Rights.
NUJ sources said a 'handful'of journalists from The Sun had become signed up to the union after it emerged News Corp's Management and Standards Committee - the body set up to investigate alleged criminal acts by journalists – had handed confidential emails of correspondence with sources and whistleblowers to the police.
The union is now appealing for more journalists who feel 'betrayed'at News International to join – claiming the company's internal News International Staff Association, which was denied union status in 2001, was trying its best for staff but 'have no chance because they are seen as creatures of Rupert Murdoch's management".
'The NUJ can defend staff at the Sun, and elsewhere in News International, and represent them against a management that seems prepared to throw them to the wolves,'said NUJ general secretary Michelle Stanistreet.
'It is not an exaggeration to say that if journalists are not allowed to offer protection to their sources – often brave people who are raising their heads above the parapet to disclose information – then the free press in the UK is dead.
"The protection of sources is an essential principle which has been repeatedly reaffirmed by the European Court of Human Rights as the cornerstone of press freedom and the NUJ shall defend it. In 2007 a judge made it clear that journalists and their sources are protected under article 10 of the Human Rights Act and it applies to leaked material.
'I will be writing to News Corp's Management and Standards Committee asking what authority it had to disclose this information. I will also be writing to staff at News international to invite them to join the NUJ."
Yesterday, The Times reported that the MSC had handed over details of some of its journalists' sources to police investigating allegations of criminality at its papers 'on the grounds that they do not deserve protection'because there is evidence they may have paid them for information.
Reuters, meanwhile, is reporting that the investigation into Sun journalists is looking into "suspected criminality over a sustained period of time" that involves 'tens of thousands of pounds".
"This is not about sources or expenses, this is an investigation into serious suspected criminality over a sustained period," a source told Reuters.
"It involves regular cash payments totalling tens of thousands of pounds a year for several years to public officials, some of whom were effectively on retainers to provide information. In totality it involves a six-figure sum."
Nine journalists have been arrested at The Sun in relation to allegations of corrupt payments to police and public officials.
News Corp declined to comment.