The National Maternity Hospital (Holles Street) in Dublin is to place a limit on the number of women who can deliver their babies there next summer and once the quota is reached, patients will be told the hospital is "booked out".
Labour's health spokeswoman, Ms Liz McManus, said the cap would create "serious difficulties" for pregnant women and had caused "serious alarm" among GPs for the welfare of their patients.
Dr Declan Keane, the master of the NMH, wrote to obstetricians last week telling them the hospital had decided to limit the number of patients booking in for the peak months of June, July, August and September next year.
He said the increase in deliveries in these months was "often coupled with a reduced staff complement".
The new procedures mean patients booking in to have their babies delivered at the hospital will have to go through the NMH's central booking agent and will be given a booking reference number. Patients will not be accepted for a clinic appointment without this number, doctors were told.
"This new system applies to all months [from October 18 th], but there will be a cap placed on the months of June, July, August and September 2005. When the quota has been reached for the capped months, the patient will be informed that the hospital is booked out, regardless of previous history.
"Unbooked emergency obstetric patients who present to the hospital will be seen, however, this does not guarantee a booking," Dr Keane wrote.
Labour's health spokeswoman, Ms Liz McManus said the development was "totally unprecedented" in the history of the maternity service and that mothers !are now being effectively told when they can and can not have their babies".
"Even this warning will have come too late for some women who are already pregnant and who will be due to deliver in the months of June and July next year.
"I understand that the difficulties arise from a shortage of staff during the summer months, but this cannot possibly justify denying pregnant mothers a bed and the opportunity to safely deliver their child," she said.
Ms McManus said the Minister for Health, Ms Harney, must "intervene immediately" to ensure that there is sufficient capacity to cater for all expectant mothers.
"Steps must be taken to ensure that there is sufficient staff to allow the hospital to function at the level required all year round. Priority must be given to patients from the catchment areas served by Holles St."
She said a "proper system" should be introduced to ensure a level of coordination between the three maternity hospitals in Dublin.
"Should a hospital, for any reason, be unable to cater for a patient there should procedures in place to channel her to an alternative hospital. The Minister for Health simply cannot allow a situation to exist where women will be turned away from the hospital that has traditionally catered for their needs and left to their own devices to find an alternative."