Excessive pressure on A&E consultants has the potential to put patients at risk, doctors are warning.
The College of Emergency Medicine said increased demand and the complexity of work was causing staff sickness and burnout.
Its survey of just over 1,000 UK consultants found six in 10 thought their jobs were unsustainable.
It comes amid mounting concern about the ability of A&E units to cope this winter.
At the start of this year, the NHS in England missed its target of fewer than 5% of patients waiting more than four hours, as waiting times hit their highest rates for nine years.
Ministers have since announced a £500m bailout fund for the next two winters to help those hospitals with the most acute problems.
The college has always maintained that a shortage of doctors is a contributory factor to the problems.
Previous reports have highlighted the fact that A&Es do not have enough consultants.
The average is just over seven per unit when it should be 10 – and 16 for the largest hospitals – according to the college.
But now the college is warning that the shortages are beginning to affect the consultants who are in place.