Hospital bosses in England say they cannot guarantee patient safety during next week's four-day strike by junior doctors.
London's Guy's and St Thomas' NHS Trust conceded patients could be harmed as managers struggle to staff rotas.
Other hospital bosses also voiced concerns over the walkout, which will affect both emergency and planned care.
The British Medical Association has refused to exempt any services but says it has plans to protect patients.
It contrasts with the approach of both the Royal College of Nursing and ambulance unions, which both excluded key emergency services from strike action.
Instead, the BMA has said it will meet trade union requirements for life-and-limb cover to be provided by considering pulling junior doctors off the picket line if individual hospitals report lives are in immediate danger during the actual strike, which runs from 07:00 BST on Tuesday to 07:00 BST on Saturday.
The BMA is after a 35% pay rise to make up for 15 years of below-inflation wage rises, but the government has called the claim unrealistic.