The Civil and Public Service Union has given the Government until Monday to respond to a long-running complaint over pay differentials of up to £100 a week between civil servants and gardai performing identical clerical duties in Garda stations.
The union's deputy general secretary, Ms Rosaleen Glacken, says if there is no response by then a complaint will be lodged with the Director of Equality Investigations.
The complaint comes as a new report recommends that 600 extra posts currently filled by gardai be worked by civilians.
The report, which was prepared for the Government strategic management initiative steering group, is expected to go to the Cabinet over the next few weeks.
If it is adopted it will increase the number of posts held by civilians to 1,400. The basis of the CPSU complaint is that its members, over 99 per cent of whom are women, are suffering gender-based discrimination.
The Association of Garda Sergeants and Inspectors is known to be hostile to the claim which it sees as threatening some of its members' posts.
The Garda Representative Association is less opposed as it believes pressure to put civilians in many posts is due largely to a desire to cut costs.
However, the GRA is expected to resist the loss of so many posts, some of which are filled by members of the force who are no longer able to perform regular Garda duties and would otherwise be forced to accept early retirement.
Some of the posts are also used to create promotional outlets for gardai in smaller specialist areas, such as forensics.
Yesterday, the general secretary of the Garda Representative Association, Mr P.J. Stone, said his members were not opposed in principle to posts being worked by civilians where this was relevant.
However, the approach of the civil service unions seemed to involve "trying to create career structures for their people and ignoring career structures for our people".
He said no decisions would be possible before the GRA had had a chance to examine the findings of the Garda resource management project, which is looking at the optimum deployment of gardai for crime-fighting purposes and is due to go to the Garda Commissioner in the near future. Mr Stone said this report would have major implications for Garda staffing levels.
The civil service unions have welcomed the steering group report on civilianisation. Ms Glacken says 10 test cases will be taken initially from a wide range of work locations. If successful it would free gardai for other duties and provide significant promotional opportunities for clerical officers.
The deputy general secretary of the Public Service Executive Union, Mr Tom McKevitt, said his union was "supportive of the process and looks forward to its implementation".