If the threat by the Garda Representative Association (GRA) to call a day of disruption to policing services goes ahead next Friday, traffic on the first day of the May bank holiday could be severely affected. Members of the 8,500-strong garda rank are being advised to call in sick for 24 hours as part of the protest to put pressure on pay negotiations with the Government.

It will be the first serious industrial action by members of the force. Under the 1928 Garda Siochana Act, officers are forbidden to strike. The "sickness" ruse, already nicknamed "blue flu", is an attempt to circumvent the anti-strike clause.

Until last Wednesday the threat was not being taken seriously, but the massive turnout (just under half of the 8,500 officers of garda rank participated) for the march on ail Leinster House has shaken Garda management and the Department of Justice, according to senior sources.

The march and further threatened industrial action are seen as damaging not only to the image of the force but to the image of the State. A stable, efficient police force is generally seen as one of the indicators of the health of the State. Senior sources say there was acute embarrassment at Government and Garda Commissioner level about the march.

There is also a degree of scepticism about the GRA's intentions. According to senior sources, the pay negotiations had been proceeding fairly well.

The GRA is seeking a reopening of the last round of public sector pay negotiations because it found its pay levels had been surpassed by the settlements for nurses and prison officers, jobs which were traditionally paid less than policing.

In the initial negotiations under the Programme for Competitiveness and Work (PCW) the GRA and the next-largest staff association, the Association of Garda Sergeants and Inspectors (AGSI), opted for a minimum pay increase in return for the inclusion of all garda allowances as part of pensionable salary.

This seemed an attractive deal, especially to older officers, as up to a third of a garda's pay can take the form of allowances, which virtually every officer receives.

However, after witnessing the major basic salary increases received by prison officers, teachers and nurses, gardai embarked on a campaign to have their better-paid basic salary ratios restored.

Previously, garda pay was at least 2 per cent higher than that of nurses and prison officers but it fell behind after the PCW was completed. The AGSI says gardai are now 10 per cent behind nurses and 35 per cent behind national teachers.

The present Government agreed to reopen the PCW for the GRA and AGSI negotiators and talks began before Christmas. It was expected that the two unions would receive a stand-alone offer of around 8 per cent and that up to a further 6 per cent would follow if they agreed a productivity package. This package would entail new rostering arrangements and the introduction of a performance development scheme and computerised IT systems.

With agreement on productivity the offer under the PCW would work out at around 14 per cent overall and re-establish the pay scale ratios between gardai, the prison officers and the nurses.

Last weekend the Government side had offered 7 per cent and seemed to be close to what was generally accepted as their upper limit standalone offer. AGSI and other senior ranks' associations seemed satisfied that the overall package would re-establish the previous parities and would be good for the force.

However, the GRA decided to go ahead with its action, causing both surprise and alarm.

The GRA dismissed the 7 per cent offer and said the march on ail Leinster House was "only the start" of a campaign of action. However, a number of officers, from garda rank up to senior management, said this week they felt there were other motivating factors.

The GRA has been in almost permanent internal dispute for the past four years since it first accepted the minimum PCW offer. It split in 1994 and only rebuilt itself at the start of this year when the 2,500 breakaway members rejoined.

In the interim there were remarkable scenes of conference shouting matches, pickets and private security men guarding one GRA conference in Galway.

Next month, the GRA holds its first annual conference as a reunited association in four years. According to some association sources, a culture of militancy and controversy has come to be expected and is even encouraged by some figures.

The concern among Garda management - and Government - is that having embarked on this path, staff representatives may be unable to avert a worsening industrial atmosphere in the force.