NHS Grampian's diversion of some ambulances away from Aberdeen Royal Infirmary (ARI) is a "major warning light" for the health service, a union says.
BMA Scotland said doctors were "extremely concerned" after a critical incident was declared by the board on Thursday over very high demand on its services.
Patients requiring immediate life saving care will still be dealt with at ARI, but other patients will be taken to other hospitals where clinically appropriate.
Health Secretary Neil Gray said the Scottish government was working urgently to help mitigate pressures on the hospital.