Southampton General Hospital has been named as one of 17 NHS hospitals that are understaffed.
The Care Quality Commission (CQC) has issued the hospitals with warnings after carrying out inspections as recently as November last year.
Each was told it did not have enough staff to keep people safe and meet their health and welfare needs.''
Labour - which released the information - said the findings reflected a "toxic" combination of cuts and reorganisation.
But the government said the number of NHS clinical staff had risen since 2010.
Health Secretary Jeremy Hunt said he expected "swift action" to be taken by those named.
"There can be no excuse for not providing appropriate staff levels when across the NHS generally there are now more clinical staff working than there were in May 2010 - including nearly 5,000 more doctors and almost 900 extra midwives.
"Nursing leaders have been very clear that hospitals should publish staffing levels and the evidence to support them twice a year. We fully support this and will put an extra £12.5 billion into the health service by 2015."
Labour claims nursing numbers in England are down nearly 7,000 since the coalition came to power. It says providers could not provide the standards of care everyone wants to see if they were overstretched.
Shadow health secretary Andy Burnham said: "The Care Quality Commission has warned that one in six hospitals are operating below adequate staffing levels, and, today, we can see for the first time the hospitals where cuts have gone too far.
"The public has a right to know if their local hospital is taking risks with staffing levels.”
Care Quality Commission staff inspected each hospital, but not every ward was visited.
Katherine Murphy, the chief executive of the Patients' Association, added: “It is a deeply worrying picture: these are just the parts of each hospital thhat the inspectors have seen and there is no reason to think it will be any different on the wards they have not visited.''