Staffing shortages are stretching specialist baby care units to the limit, campaigners say.
Premature baby charity Bliss found just 20% of UK hospitals had enough staff to meet recommended care guidelines.
The study of 194 neonatal units showed that more than 50% had been forced to close to new admissions during a five-month period because of shortages.
The charity said it was shocking that such problems were persisting. The government insists care is safe.
Each year, more than 80,000 babies - 10% of the total born - need specialist care in a neonatal unit - usually because they are underweight or premature.
Level one - Known as special care baby units, these are the least intensive and most common type of neonatal care. Their work normally involves monitoring breathing and heart rate. Recommended staffing ratio is four babies to one nurse
Level two - Babies are admitted into high dependency units if they weigh less than 1,000g (2lbs 3oz). These children will usually require help breathing and there should be one nurse for every two babies
Level three - Only the most severely ill babies end up in these intensive care units. They are highly specialised with babies requiring ventilation and constant care to keep them alive. Staffing ratio should be one-on-one
Bliss surveyed all 213 British hospitals with neonatal units about care, from April to September 2007, and 91% of those asked responded.
They found that only 21% of the hospitals which responded had enough staffing to provide the recommended nurse to baby ratios.
These vary between one-on-care in intensive care wards to one nurse to every four babies on the least-intensive special care baby units.
To meet the recommended guidelines the charity said it believed an extra 1,700 neonatal nurses were needed in addition to the 6,500 already employed.
More doctors were also needed, it added.
But it warned there had been a lack of progress made in neonatal care over the years.
This is the fourth report the charity has produced highlighting the issue. Over the last year fewer than 200 extra nurses have been recruited.
The charity also polled more than 300 parents who complained of being transferred between hospitals - sometimes over long distances - because of staffing shortages.
Bliss chief executive Andy Cole said: "Professionals are increasingly being stretched to the limit.
"Staffing shortages are all too apparent on units and the care of our most vulnerable babies is being compromised.
"No other critical care service would permit the capacity and staffing levels seen on special care baby units."
David Field, president of the British Association of Perinatal Medicine, agreed the staff shortages were worrying.
He added: "We would like neonatal care to be given the same kind of priority that other specialist services such as cancer care or A&E have received."
But a Department of Health spokesman said there was no evidence that services were "unsafe".
"We are committed to providing mothers and babies with safe, high quality neonatal services and have made neonatal services a top priority for the NHS.
"However, we recognise there is still more to do."
Meanwhile, a spokeswoman for the Scottish government said that a review of neonatal care was currently taking place.
She added: "The Scottish government is committed to providing the best possible care for premature babies."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.