The Safe Staffing Alliance (SSA) says wards often have just one registered nurse looking after eight patients.
The SSA says research shows when nurses are asked to look after more than eight patients there is an increased in risk of harm or death.
The warning comes as a poll for the Sunday Mirror and the Nursing Standard journal found that more than three quarters of nurses believe a scandal similar to that in Mid-Staffordshire could happen again.
In the poll of 2,000 nurses, 40% said on their last shift there were not enough staff to provide a safe level of care.
More top news
It will be a cloudy evening with patchy rain at first, with most places becoming dry overnight. Turning wetter tomorrow.
Apart from the odd spot of rain at first most of the night will be dry but outbreaks of showery rain will spread in tomorrow afternoon.
A Ukip parliamentary candidate for a Birmingham seat has been standing and waving at cars on a busy traffic junction for 13 hours a day.