More than one in 20 nursing posts are vacant, nursing leaders have warned.
Freedom of Information requests conducted by the RCN found that hospitals average vacancy rates were around 6%.
Extrapolating the figures from 61 hospitals, the College said this would amount to nearly 20,000 full-time equivalent nursing, midwifery or health visitor vacancies.
As a result of the NHS's efficiency drive, many trusts froze vacancies or held posts vacant, RCN head of policy Howard Catton said.
In the aftermath of the care crises at Mid Staffordshire NHS Foundation Trust and Winterbourne View care home, employers have "turned a corner" and have recently started recruiting again, he said.
But the supply of nurses has "reduced", he added.
A 15% reduction in the number of student nurses combined with an ageing workforce and a number of staff leaving the profession because of stress and excess workloads are all factors contributing to a "workforce crisis", Mr Catton said.
He went on: "Freezing vacancies and holding posts vacant were a very consistent, commonly recurring strategy to make efficiency savings.
"Clearly the events at Mid Staffs and Winterbourne View have shone attention on the importance of nurse staffing levels.
"As a consequence over recent months we have seen some trusts starting to unfreeze vacant posts and to start to raise their establishment level and look to recruit more nursing staff.
"However, when they have gone to market to try and recruit they have found that difficult because we have reduced the supply of nurses and as a consequence a number of places have had to go and look for nurses overseas."
The RCN's latest Frontline First report states that, as a result of the nursing shortage in the UK, 22% of trusts said they have recruited nursing staff from abroad.
The report states that the Department of Health stopped collecting vacancy data in 2011, when vacancy rates for nurses stood at 2.5%.
Mr Catton called on ministers to resume monitoring vacancy rates.
The College also made a series of recommendations including a mandatory requirement for safe staffing levels.
Dr Peter Carter, chief executive and general secretary of the RCN, said: "Understaffing remains a real issue across the NHS, and we know that many Trusts are down to the bone in terms of the number of frontline nursing staff they have due to cutting posts to save money.
"Unsafe staffing levels have been implicated in a number of high-profile investigations into patient safety. We call on employers in the NHS to put an end to boom-and-bust workforce planning and develop clear standards to ensure safe staffing levels are met, supported by robust inspection based on reliable data."
Health Minister Norman Lamb said: "Nursing leaders have been clear that hospitals should publish staffing details and the evidence to show the numbers are right for the services they deliver. Patient safety experts agree that safe staff-patient ratios should be set locally. We will be announcing more on our plans to guide staffing decisions in our full response to the Francis report later this autumn.
"Overall, the number of clinical staff in the NHS has increased by nearly 4,100 and the number of admin staff has fallen by 22,800. The Chief Inspector of Hospitals will be able to take action if Trusts are found to be compromising patient care by not having the right number of staff on wards."