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
After a wet and windy start, rain will clear to the north to leave it dry, with easing winds but largely cloudy skies.
Early rain will clear northwards during the morning, allowing drier conditions behind.
Thousands of patients have had their operations and appointments cancelled despite a strike by junior doctors being called off.