The rise in hospital nurses in England has been dwarfed by a jump in patients, a study suggests.
The Royal College of Nursing research points to data showing the nurse workforce has increased by 4.6% in five years.
But hospital admissions have jumped by 12.3% - nearly three times the rate.
The union says this shows more needs to be done to ensure safe staffing, but the government says the rising NHS budget will ensure high-quality care.
What has the RCN found?
The fact there are shortages of nurses is already well established - latest figures suggest one in nine posts.
But the government often counters that by pointing out the number of nurses working in the NHS is increasing.
Official data shows there were 224,000 hospital nurses on average during last year - up by 4.6% in five years.
But to put this in context, the RCN also looked at how much the number of admissions to hospital had gone up by.
Last year there were 14.2m, which is 12.3% higher than it was five years ago.
RCN general secretary Dame Donna Kinnair said the data should act as a "stark warning".
"Staffing shortfalls are never simply numbers on a spreadsheet - they affect real patients in real communities."
She said there needed to be a new law to ensure safe staffing, as has been introduced in Wales and Scotland.
She also said she wanted a national body to take on responsibility for properly planning the nursing numbers needed in the future.
What is the government doing?
The government says ensuring there are enough staff available for the NHS is a key priority.
The number of training places for nurses is in the process of being increased by 25%, and there are campaigns under way to attract nurses who have left the profession back to the NHS.
New roles to support nurses - called nursing associates - are also being created.
This drive is being supported by the increased investment in the health service, the government said.
One of the initiatives being funded with that money in the coming years is a £1,000 personal development budget over three years for all nurses, while a detailed workforce plan is also expected to be published soon.
"The safety of patients is paramount" a Department of Health and Social Care spokesman added.