Around 150,000 teachers, gardaí, nurses, doctors, prison officers and other frontline staff are excluded from the government's generous public-service early-retirement scheme which was formally launched by finance minister Brian Lenihan last Friday.

Announced in last April's supplementary budget as a means to cut soaring public-service numbers, the scheme allows public servants aged 50 and over to retire without any actuarial reduction in their pension.

But under the scheme's rules laid down last week, those opting to leave early cannot be replaced. This means the offer cannot be open to those providing an irreplaceable frontline service such as teachers and medical staff.

This will come as a blow to older teachers in particular, who had a special teacher-specific early-retirement scheme scrapped under the previous budget last October.

Under another rule of the scheme drawn up by Finance, public servants who can avail of early retirement in their jobs cannot apply. This rules out gardaí, prison officers, firefighters and psychiatric nurses who can retire on a full pension at 50 as long as they have 30 years' service.

Those excluded from the scheme amount to around half of the 300,000 currently employed in the public service. With the public-service paybill hitting €20bn this year or almost 40% of day-to-day expenditure, this will significantly reduce the cost-saving potential of the scheme for Lenihan.

Public-service unions Impact and the Ahcps, which represent senior and professional public servants, many of whom would be eligible for the scheme, have pointed to some possible legal difficulties with it.

The scheme will be left open until 1 September, after which it will be reviewed in the context of next year's budget.