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.
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A dry start, with sunny spells developing. However turning cloudier again later, with showers possible by the evening.
After a fine evening with some late sunshine to the end the day, a largely dry and fairly warm night will follow with clear spells.
Any remaining showers will fade, to leave it dry overnight.