NHS staff in England are working on the "edge of safety" as rising demand is outstripping the increasing numbers being employed, health bosses say.
There are now 6% more staff than there were three years ago, but demand for services has risen by three times as much in some areas.
NHS Providers, which represents health chiefs, said staff shortages was now the number one concern in the NHS.
But ministers insisted there were plans in place to tackle the problem.
Over the past year, Health Secretary Jeremy Hunt has announced rises in the number of training places for both doctors and nurses.
The Department of Health said this represented the "biggest ever expansion of training places" and would help ensure the NHS had the staff it needed.
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But Saffron Cordery, of NHS Providers, said there was no guarantee this would work as there was no over-arching "coherent or credible" strategy.
She said her members were really worried about the shortages on the front line, which was leaving staff with "undoable" jobs.
"They are now working on the edge of safe services. We are seeing so much pressure on the front line."
She added the prospect of Brexit was just making things worse, with EU staff facing "much uncertainty" about their jobs and future careers in the NHS.
Just last week, figures from the Nursing and Midwifery Council showed the number of EU nurses and midwives registered to work in the UK had fallen by 2,700 in the past year, to just over 36,000.
A staffing crisis?
The report by NHS Providers found the total number of staff working in the NHS had risen by 6%, to 1 million, between 2013-14 and 2016-17.
Bu the same period had also seen the following rises in demand for services:
- diagnostic tests: 19%
- visits to accident and emergency units: 7%
- emergency admissions: 10%
- ambulance calls: 15%
- GP referrals: 11%
Labour shadow health secretary Jonathan Ashworth said: "This is a damning report. The staffing crisis facing our NHS reflects a fundamental failure at national level on workforce strategy.
"In the upcoming Budget, the government must fully fund the scrapping of the pay cap for NHS staff and bring forward wider funding to put our NHS on a sustainable footing."
Royal College of Nursing general secretary Janet Davies said: "Ministers can no longer dismiss warnings of this kind."