The care watchdog has demanded improvements at a privately-run Devon ambulance service.
The Care Quality Commission raised concerns about the Four Counties Ambulance Service, based in Wilmington.
It has issued a formal warning to provider Rowan Griffiths for failing to protect the safety and welfare of clients.
Inspectors visited the service in November after concerns were raised and found that it was not compliant with seven essential standards of quality and safety.
The provider was singled out by the CQC for safety failings, quality monitoring and staff recruitment policies.
Ian Biggs, deputy director of CQC in the South said: "The law says that these are the standards that everyone should be able to expect.
"This warning sends a clear and public message that Mr Griffiths needs to address these issues as a matter of urgency or face serious consequences.
"We will return in the near future and if we find that this ambulance service is not making the required progress we won't hesitate to use our legal powers to protect the people who use it."