Neonatal units in England are having to close their doors to new admissions once a week on average because of staff shortages, a watchdog says.
There are 180 neonatal units in England
The National Audit Office report said there were 459 vacancies for neonatal nurses - 9% of the total workforce.
Inspectors also warned many of the 180 units in England were too full to give the best care to sick babies.
Campaigners said improvements were needed, but the government said measures were being taken.
The report said a third of units were operating above the 70% capacity set out as a guideline by the British Association of Perinatal Medicine.
Another three units were operating above 100% capacity, meaning there were more babies in cots than there were trained staff to care for them.
The report said: "These high occupancy rates could have consequences for patient safety, for example due to the increased risk of infection or inadequate levels of care."
One in 10 babies born - about 60,000 a year - need specialist neonatal care for reasons such as being born premature, low birth weight or suffering from a condition like a heart defect.
However, the report did praise the reorganisation of neonatal units into 23 regional networks - not all units have the most specialist intensive care facilities and therefore share them with neighbouring hospitals.
The NAO found the networks had cut the need for long-distance transfers, but also warned only half had specialist transport 24 hours a day, seven days a week for those babies that needed moving.
The NAO also found wide variations in the death rates of babies.
In 2005, the South West Midlands network had the highest death rate at 4.8 babies per 1,000, while the figure was 1.8 in Surrey and Sussex.
These differences were likely to be down to a combination of quality of care and socio-economic factors, the report added.
Government maternity tsar Dr Sheila Shribman said there was "nothing more important than the care and support we offer sick babies".
She added: "The networks are working hard to recruit and retain staff as well as introducing new ways of working to free up specialist nurses to deliver the cot-side care they were trained for."
But a spokeswoman for Bliss, the premature baby charity, said: "The report reveals some serious problems in the service that cares for sick and premature babies and confirms that babies are not receiving the same level of care as children or adults."
And she added there were concerns there was no "strategic" plan to deal with the expected rise in the number of births in the coming years.