The SDLP is strongly supporting the Sunday Tribune in its refusal to comply with PSNI demands that the paper's Northern Editor hand over phones, computers and other material relating to stories on the Real IRA.

SDLP leader Mark Dur­kan, along with the Alliance party's EU candidate Ian Parsley, and Green candidate Stephen Agnew, have backed a campaign supporting Suzanne Breen in her legal battle with police.

The campaign has attracted significant national and international support. It has been endorsed by journalists, writers, civil liberties groups, trade unionists, and arts and cultural figures.

Last week, the SDLP attempted to table motions in the House of Commons and the Stormont Assembly supporting Breen, saying the PSNI's actions could seriously undermine investigative journalism.

SDLP Assembly member, Alex Attwood, said he was stunned when told that neither chamber could debate the matter because it was sub judice (under judgement).

Attwood said: "Weeks ago, the Assembly debated Ian Paisley jnr's right to protect sources relating to the Billy Wright murder inquiry. The Assembly debated the Omagh bomb during the civil action by Omagh relatives.

"We debated Raymond McCord jnr's murder during that criminal investigation. Nobody claimed those matters couldn't be discussed because they were sub judice."

Attwood said the decision regarding the Breen motion had been made by the Assembly's "business office" and that the decision to ban the Commons' motion had been taken after Westminster consulted with the Northern Ireland Office (NIO).

"Why can the Assembly debate and support Ian Paisley jnr's right to protect sources, but can't do the same for Suzanne Breen?" he asked. "It's very inconsistent – indeed, it might be even more than inconsistent. This must be investigated thoroughly."

Attwood said the SDLP fully supported the PSNI investigation into the murder of two soldiers at Massereene but believed a court order forcing Breen to hand over journalistic material "would put investigative journalism and journalists at risk".

He said: "Reporters have a right to do their job. An independent media is essential." The SDLP will be resubmitting reworded motions to Westminster and Stormont.

The north's two main parties, the DUP and Sinn Féin, have bizarrely remained silent on the Breen case. The DUP strongly defended Ian Paisley jnr's right to protect his sources with first minister Peter Robinson even giving court evidence on his behalf.

Sinn Féin has traditionally opposed censorship and state infringement of press freedom but has so far failed to support Breen. Jim Boumelha, president of the International Federation of Journalists, which represents 600,000 reporters in 123 countries across the world, condemned the PSNI's actions and praised the Sunday Tribune.

Breen will address a National Union of Journalists (NUJ) rally in London in support of the Sunday Tribune's stand for source protection on 26 May. Other speakers will include Channel 4's chief correspondent, Alex Thomson.

The full hearing of the PSNI case against the Sunday Tribune opens in Belfast on 29 May. The NUJ is organising a picket outside the court. The NUJ will simultaneously hold a protest outside the NIO's London offices at Milbank.

A Facebook support group and petition, along with an NUJ petition, has already attracted over 500 signatures within days. Support messages from organisations and individuals can also be sent to sbreen@tribune.ie