Premature baby charity Bliss discovered 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 commented it was shocking that such problems were persisting, while the government insists care is safe.
Bliss surveyed all 213 British hospitals with neonatal units, about care, from April to September 2007, and 91% of those asked responded.
It found that only 21% of respondents had enough staffing to provide the recommended nurse to baby ratios.
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.
This is the fourth report the charity has produced highlighting the issue. Over the last year fewer than 200 extra nurses have been recruited.
Bliss chief executive Andy Cole said: “Staffing shortages are all too apparent on units and the care of our most vulnerable babies is being compromised.
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.”
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.