The news comes as Murdoch is due to visit London today to meet Sun journalists, after promising to continue publishing Britain’s top-selling daily despite the controversy.
The Australian-born tycoon shut down The Sun’s weekly stablemate, the News of the World, in July after it became embroiled in a phone-hacking scandal.
Police arrested five Sun journalists last weekend on suspicion of bribing public officials, after receiving information from the Management Standards Committee (MSC) set up at Murdoch’s News Corp to investigate allegations of wrongdoing.
The arrests caused outrage in the newsroom and the National Union of Journalists (NUJ) said it was now looking at how to defend staff “against a management that seems prepared to throw them to the wolves”.
“We have been approached by a group of journalists from The Sun. We are now exploring a number of ways to support them, including discussing legal redress,” said NUJ General Secretary Michelle Stanistreet.
Stanistreet said the issue at stake was the protection of sources, and suggested the journalists might use European human rights laws.
“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,” she said.
News International, Murdoch’s British newspaper unit, refused to comment on the possibility of staff legal action.
The tycoon is flying to Britain this week to manage the latest crisis in his media empire.
In an email to staff when the journalists were arrested on Feb 11, Murdoch executive Tom Mockridge said the proprietor had given him a “personal assurance... about his total commitment” to publishing The Sun.
Ten current and former senior reporters and executives at The Sun have been arrested since November over alleged corrupt payments to public officials.
Murdoch is expected to offer further reassurances in a direct address to employees during his visit to the paper’s offices in Wapping, East London over the future of the paper.
Five Sun journalists — including the deputy editor, picture editor and chief reporter — were held for questioning by Scotland Yard officers on Saturday, on suspicion of making improper payments to police and other public officials. They have all been bailed.
The latest arrests have provoked criticism that the Metropolitan Police was being heavy-handed and that the MSC had identified some journalists’ confidential sources to detectives.
Trevor Kavanagh, The Sun’s associate editor, said: “There is unease about the way some of the best journalists in Fleet Street have ended up being arrested on evidence which the MSC has handed to the police.”
The NUJ’s Michelle Stanistreet, said: “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 investigation into The Sun has disclosed evidence suggesting that tens of thousands of pounds a year were paid to public officials for information.