The NHS in England is drastically short of the nurses it needs, with 40,000 posts unfilled, figures suggest.
The total is double what it was in 2013 – and means one in nine positions is now vacant, according to the analysis by the Royal College of Nursing.
The union said the situation was dangerous, blaming the stress of working in the NHS and the cap on pay rises for the problem.
But the Conservatives said plans were in place to tackle the issue.
The party has said the extra money being invested – an average of 1% a year between 2010 and 2020 – is enabling ministers to ensure patient safety is prioritised and, despite the vacancies, the number of nurses employed is still rising.
Between 2010 and 2016 the numbers employed have risen by 2% to just over 300,000 full-time nurses.
The scale of the problem
The RCN, which is holding its annual conference in Liverpool, does not dispute this, instead it has looked at how many nurses NHS trusts needed to employ, but cannot.
It relied on freedom of information requests to obtain data from all types of NHS trusts for the end of 2016 and received responses from three-quarters.
They suggested on top of the nurses employed there were another 40,000 posts unfilled across the whole health service.
This equates to a vacancy rate of over 11% and compares to a total of 20,000 when the RCN last carried out similar research in 2013.
The rate was greatest in mental health services where more than 14% of posts were empty, compared to close to 8% in specialist services, such as cancer and heart hospitals.
Official figures in Scotland and Northern Ireland show a much lower vacancy rate at around 4% in each nation. Figures were not available for Wales.
Feedback from senior nurses showed four in five felt the NHS was only able to keep services running because of the “goodwill” of staff.
The research also suggested the NHS was increasingly turning to lower skilled staff, such as health care assistants, as they struggle to recruit enough nurses.
Pay cap is being blamed
The RCN said the cap on pay rises, which is continuing until 2019, was a major factor. Results of a poll on strike action over this is due on Sunday.
RCN general secretary Janet Davies said the shortages were “dangerous” as they were now risking patient care.
“A lethal cocktail of factors in the NHS has resulted in too few registered nurses and patient care is suffering.”
She said the situation could get even worse in future years because the NHS was so reliant on nurses from Europe and numbers could be affected by Brexit.
Ms Davies said it was now time to introduce strict rules on safe staffing, while lifting the pay cap. These are both things Labour has promised if it forms the next government.
It comes after NHS Providers, which represents trusts, warned last week that retaining staff was a major concern for NHS bosses, with reports of low-paid staff leaving to work in supermarkets.
A nurse’s story: ‘I have to get out’
Sam (not her real name) has only been working as a nurse for 18 months, but already she is looking to leave the profession.
“The pay is an issue, but it’s the pressure that is the key thing. I work in an emergency department and we are constantly understaffed. I should have not more than four or five patients, but it is always more than that. And we rely heavily on agency staff and that puts more pressure on – and I am newly qualified.
“It means I can’t provide the safe care I want to. I am rushing from patient to patient. We are having to discharge them before they are ready. It is really upsetting.
“The other week an elderly patient came in from a nursing home, but the doctors could not attend to her. She had a cardiac arrest. I had to perform CPR. I just can’t keep working in these conditions. I need to get out.”
It’s a scandal, say politicians
Liberal Democrat health spokesman Norman Lamb said the number of unfilled posts was a “scandal”.
“The Conservatives are bleeding the NHS dry,” he added.
Meanwhile, Labour’s shadow health secretary Jon Ashworth described it as “terrible news” for patients.
But the Conservatives have argued that the figures put forward on pay ignore the fact that half the workforce receive annual progression-in-the-job rises which bring the yearly rises to 3% on average for those with them.
A spokesman added that introducing safe staffing levels could undermine the “judgement” of senior doctors and nurses, but ministers remained determined to “ensure that standards of safety continue to rise”.
He said a Labour government would put NHS services at risk because of its bad economic policies.