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
Hundreds of steelworkers will march on Wednesday to highlight the crisis in their industry.
Leicester City star Jamie Vardy will marry his fiancee Rebekah Nicholson in a lavish ceremony on Wednesday.
Leicester City striker Jamie Vardy is set to marry his partner Becky Nicholson today after moving the date due to a clash with Euro 2016.