The number of vacant nursing posts has doubled over the past few years amid a "lethal cocktail" of rising demand and falling pay, the government is being warned.
The Royal College of Nursing said safe staffing levels should be enshrined in law so nursing directors do not have to fight for funding.
Figures obtained by the RCN from NHS trusts in England revealed that one in nine nursing posts are unfilled and suggests that care providers have increasingly hired fewer registered nursing staff.
The number of vacant nursing posts has doubled since 2013, said the RCN.
A Freedom of Information request by the college showed that two-thirds of NHS hospital trusts in England planned for a greater proportion of nursing support staff last year than a year earlier.
Janet Davies, general secretary of the RCN, said the move leaves the Government open to the accusation of offering "nursing on the cheap" in place of recruiting and retaining registered and experienced nurses.
She will tell the RCN's annual conference in Liverpool: "A lethal cocktail of factors in the NHS has resulted in too few registered nurses and patient care is suffering.
"Pressure and demand has spiralled upwards at the very moment nurses' pay headed the other way. They stay behind after 12-hour shifts to give patients extra care and go home exhausted and sometimes in tears. Too many now feel no alternative but to leave nursing.
"There is no certainty about the next generation of UK nurses joining either - deterred by low pay, pressure and new training costs - so the Government desperately needs to keep the experienced ones we have.
"When finances are tight, nursing budgets are slashed and patients can pay the highest price. Hospitals are hiring unregistered staff and delegating jobs that should be done by trained nurses. The Government cannot allow nursing on the cheap.
"Ministers must draw a line under this false economy and address safe staffing levels in new legislation. Nursing directors should not have to fight for the funding."