More than one in five British hospitals may have to close their accident and emergency units at night because they don't have enough doctors, the Royal College of Physicians has warned.

The NHS was facing a crisis caused by the demand for a round-the-clock service when there was a shortage of doctors, the college said.

Tomorrow, the health minister John Hutton is to meet Professor Carol Black, president of the college, to discuss her concerns. Cutting back on casualty services at night would be politically damaging - but ministers have ruled out the only other option, to delay implementation of the working time directive for three to six years to allow extra doctors to be recruited.

Under the European directive, doctors will be limited to a maximum of 58 hours a week from August 2004. It makes no allowance for the fact that Britain has half the European average number of doctors per 1,000 population.

Professor Black told the Health Service Journal the issue was one of the most pressing facing the NHS. A survey of emergency cover at 211 hospitals in England had concluded that 44 were at "immediate risk" and 122 had "insufficient staffing", she said.

The survey is being sent to the Department of Health but has not been published. Professor Black said: "The 44 [at immediate risk] will find it very difficult to cope and must be worrying now about how they are going to comply. At the moment they are coping because they have dedicated staff and a relatively small number of emergency admissions." She said she backed pilot projects in which nurses were being trained to take over some tasks of doctors to ease pressure on A&E units - but added: "The problem is in the middle of the night you have to have someone to make the decisions, and that is what you've trained doctors for."

A spokesman for the Health Department said the working time directive had been known about for many years. "We recognise it is a challenge and there is a genuine sense of urgency to find solutions to ensure patient services won't be affected," he said.