Patient safety and quality of care are at risk due to a workforce gap in the NHS, health leaders have said.
Workforce concerns have become the "single biggest risk facing services", according to NHS Providers, a membership body, which represents health service organisations.
The gap between the number of staff NHS bodies need and the number they are able to recruit and retain is now unsustainable, according to the latest report.
This is putting patients at risk and could undermine plans to transform the health service, it said.
"The NHS... is struggling to cope with growing and changing pressures. We have now reached a tipping point: workforce concerns have become the single biggest risk facing services," the report states.
"The gap between the workforce that providers need and the staff they are able to recruit and retain is now unsustainable, putting patient safety and quality of care at risk. It may also undermine much-needed schemes to transform and modernise 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.
Staff shortages have already led to closures of some services and put extra pressure on existing staff, the report added.
"The workforce gap is most obvious in respect of clinical staff, resulting in closures of some services, for example A&E and children's services, restricted opening hours for others, and pressure on staff required to put in extra hours as they seek to maintain quality of care," the authors added.
It said that while the number of clinical staff has increased over the last seven years, this growth has "not kept pace with rising demand for services".
The body, which is holding its annual meeting in Birmingham, also said that it is unclear whether tactics aimed at improving home-grown NHS staff will be enough to meet growing demand.
A new poll of 149 chairmen and chief executives of NHS trusts and foundation trusts 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.
The survey also found that 85% said it would be important to recruit staff from outside the UK in the next three year.
But uncertainty linked to Brexit was cited as a challenge in the recruitment of overseas health workers.
NHS Providers chief executive Chris Hopson (pictured) said: "The staff and skills shortages we now see reflect a fundamental failure at national level on workforce strategy.
"We don't have enough staff with the right skills and we're asking far too much of our existing staff.
"NHS trust leaders are telling us there are no quick fixes to improve the supply of UK-trained staff, and the outlook for international recruitment is uncertain.
"These problems have developed in plain sight, which clearly shows the existing approach is flawed.
"The Government must deliver certainty for EU staff. It should reassure them that their commitment to the NHS is greatly valued and will continue to be welcomed.
"It should also provide assurance on immigration policy so trusts can continue to recruit overseas while we strengthen our workforce here."
Labour's shadow health secretary Jonathan Ashworth said: "This is a damning report cataloguing failures of ministers on workforce planning. As NHS Providers makes clear, the staffing crisis facing our NHS reflects a fundamental failure at national level on workforce strategy."
A Department of Health spokeswoman 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 the future of EU nationals is a top priority in the Brexit negotiations and we want their valued contribution to the NHS to continue - to ensure the NHS has the staff it needs both now and in the future."
Copyright (c) Press Association Ltd. 2017, All Rights Reserved. Picture (c) BBC / PA Wire.