The analysis, based on data released on the NHS Choices website, also confirms hospitals are putting more unregistered support staff on shift to cope with the shortage of registered nurses.
This substitution is particularly prevalent on night shifts, when two-thirds of the largest hospital trusts put more health care assistants on the wards than planned.
These findings support the RCN's recent research highlighting 40,000 nurse vacancies across the NHS in England.
RCN Chief Executive Janet Davies said the findings showed patients were being put at risk and called on the Government to increase the number of nurses.
“These startling figures show that, despite the Government’s rhetoric, our largest hospitals still do not have enough nurses and that is putting patients at risk.
“In light of this, the Government must redouble its efforts to train and recruit more qualified nurses and stop haemorrhaging the experienced ones who are fed up, undervalued and burning out fast.”
Janet added it was unreasonable to expect unregistered staff to fill staffing gaps.
“It is unfair on the healthcare assistants too – they should not be left in a situation they have not been trained to handle.
“Nurses have degrees and expert training and, to be blunt, the evidence shows patients stand a better chance of survival and recovery when there are more of them on the ward.
“Patients can pay the very highest price when the Government encourages ‘nursing on the cheap’.”