The Morris tribunal has ordered that two senior politicians must identify their sources for serious allegations against certain members of the Garda Síochána.
The TDs at the centre of today's judgment, Mr Jim Higgins of Fine Gael and Labour's Mr Brendan Howlin, are both former justice spokesmen in the Dáil.
The 23-page ruling was delivered by tribunal chairman Mr Justice Frederick Morris this morning. The Morris tribunal is investigating alleged corruption by a number of gardaí in Co Donegal.
Mr Higgins and Mr Howlin were approached by past or present members of the force in 2000 and later relayed the allegations made to them to the-then Minister for Justice Mr John O'Donoghue.
Both Mr Higgins and Mr Howlin were instructed by the tribunal last year to name their sources and also to hand over telephone and fax records of contacts they had.
Last week they told the tribunal they had the right under the Constitution to refuse to name the people who gave them information.
Neither was at the tribunal to hear today's ruling but lawyers for both men indicated they were to take the case to the High Court for a judicial review of the judgment. A four-week stay has been put on today's ruling.
Mr Howlin later said he was disappointed, but not surprised at the ruling. Confirming plans to contest the judgment, he maintained "the principle that is at stake here has resonance right across public administration".
He said of the allegations about the Garda that he received from an informant: "Either they are true, or they are not. The source is not absolutely essential.
"As far as I am concerned, the information given to me was privileged. I assumed it was privileged, the person who gave it to me assumed it was privileged and I must protect that right so that the next person who comes to me with concerns about wrongdoing must able to do that without fear".
Mr Higgins said: "I am prepared for any possible consequence because this judgement . . . flies in the face of the Constitution and I will take whatever steps and measures are necessary to protect the constitutional right of public representatives . . . and the right of the members of the public to come to us".
The tribunal, which opened in Donegal last year is due to begin hearing detailed allegations made by a number of witnesses next week. Its proceedings were delayed earlier this year because of the challenge on the privileged information issue.
The inquiry was set up by the Dáil almost two years ago to investigate the allegations that centre on claims about Garda inquiries into a number of cases, including paramilitary bomb finds, arson and a series of arrests as well as the treatment of the McBrearty family in Co Donegal.