Overcrowded hospitals in England were forced to close their doors nearly 500 times to ambulances this winter.
The number of diverts was nearly double the level of the previous three winters, and has sparked concern about the pressures facing the health service.
The figures showed that there were 478 diverts in December, January and February, compared to 265 the winter before.
Hospitals said closures were a last resort, but paramedics warned it had a knock-on effect of slowing their ability to reach the sick and injured quickly.
Latest figures show that ambulance crews have failed to hit any of their response time targets since May 2015, with a third of the most critically ill patients waiting more than eight minutes for a 999 response.
Saffron Cordery, of NHS Providers, which represents hospitals, said the trend was "stark".
"Diverting ambulances to other hospitals is disruptive for paramedics and bad for patients.
"The sharp increase in the number of diversions is very worrying. Further steps will be needed across health and social care to address these pressures."
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