The public service union, Pseu, has raised concerns about the anomaly which will affect the women's rights to medical, dental, pension and other welfare entitlements.

Close to 800 civil servants work on a week on/week off basis, while many multiples of that number in the broader public service and throughout the entire workforce will also be affected by this anomaly.

Family-friendly work initiatives are designed to allow workers spend more time at home with their children. It is mainly women who take up job shares and, on top of the cut in pay, they now find they are being doubly penalised for opting to look after their children.

The anomaly has emerged in the same week that the European Court of Justice controversially ruled that it was not illegal for men to be paid more than women if they had longer service, even if they were doing the same job.

A British woman had argued that she had run up less service than a man only because she had taken time out to look after her children, something which is mainly done by women.

Yet she was paid less while doing the same job. But the court found against her and ruled service in the job can justify a higher salary.

The current PRSI anomaly only affects people on a week on/week off job-share system and not other jobshare arrangements.

It arises from the fact that, under existing legislation, a contribution week for PRSI purposes commences at the start of the tax year, on whatever day 1 January falls. Next year, this will be a Monday so the PRSI 'contribution week' will go from Monday to Sunday throughout the year.

Under the Department of Social Affairs' rules, to qualify for a PRSI contribution . . . or 'stamp' as it used to be known . . . a person must work at least one day in a PRSI contribution week. But because the week goes from Monday to Sunday, those on a week on/week off arrangement will not be credited with a stamp for the week they are off.

"If members are working such attendance patterns, they will put at risk their social welfare entitlements in 2007 unless they change to a pattern which involves a split week, " said Billy Hannigan of the Pseu.

A department spokeswoman acknowledged the problem but added that people job share "on a voluntary basis".

She added that in a recent report on social insurance systems, unions and employers did not think that the "issue warranted any overhaul of the social insurance system".