Staff shortages in the NHS pose a fundamental risk to patient safety and quality of care, according to managers of hospital and ambulance trusts across England.
The NHS is facing a recruitment crisis with an estimated shortage of at least 50,000 doctors and nurses, and the Department of Health is admitting that it does not have enough staff.
A new survey of managers at 149 trusts in England by NHS Providers, which represents health service organisations, found the majority believe workforce issues are the "single biggest risk facing services".
The report warns that the gap between the number of staff NHS bodies need and the number they are able to recruit and retain is now unsustainable, and the problem may be exacerbated by Brexit.
"The NHS … is struggling to cope with growing and changing pressures," the report states.
"We have now reached a tipping point: workforce concerns have become the single biggest risk facing services."
The authors said that the workforce gap has almost certainly widened since Health Education England reported a staffing shortfall of 5.9% or 50,000 clinical staff in 2014.
Saffron Courdery, head of policy at NHS Providers, told Sky News: "Our research shows that workforce is the biggest challenge the trusts are facing.
"Two thirds of trust leaders tell us that they are more concerned about this than the money, and 85% of them say they are really concerned about recruitment of staff now that Brexit has come into play.
"What that means in practical terms is that staff are facing a huge amount of pressure in their jobs day to day.
"What we are seeing is things like rota gaps, we are seeing staff being asked to work double shifts, we are seeing services sometimes needed to be closed because there aren't enough staff and we are seeing all sorts of pressures in different parts of the system."
The poll of trust chief executives and chairmen found that 66% believed workforce challenges were the most pressing issue facing their trust.
When asked for the biggest challenges to recruitment and retention at their trust, 60% of trust chairmen and chief executives cited work pressure and 38% cited pay and reward.
A spokesperson for the Department of Health said: "The NHS has over 12,700 more doctors and 10,600 more nurses on wards since May 2010 – but we know that we need more staff.
"That's why we recently announced the biggest ever expansion of training places for doctors and nurses, as well as being clear that European workers will be able to stay after we leave the EU – to ensure the NHS has the staff it needs both now and in the future."