Health regulators today criticised the company Serco over its out-of-hours GP services in Cornwall and the Isles of Scilly.
The Care Quality Commission (CQC) said the firm was not meeting four of the essential standards of quality and safety - providers have a legal responsibility to make sure they are meeting all those standards.
A CQC spokesman said Serco had been given 14 days to provide a report setting out how it will achieve compliance.
"CQC inspectors will revisit the service at a future date without giving any notice to check that the required improvements have been made."
The report followed inspections conducted during April and May, when a team of inspectors, joined by professional advisers, made unannounced visits to the company's call centre in Truro and five clinics where they met patients, GPs, support staff and others.
They also spoke to current and former members of staff who asked to pass on information in confidence.
The inspectors found that Serco was meeting four of the essential standards which were reviewed, but not meeting the four others.
:: Safeguarding people from abuse - inspectors concluded that people who use the service were not protected from the risk of abuse, because not all staff were trained in the safeguarding of vulnerable adults and children.
:: Staffing - inspectors found there were not enough qualified, skilled and experienced staff to meet people's needs. While the service was currently in the process of recruiting more GPs, there was a shortage of clinical staff at times. Inspectors found that on one particular weekend, some doctors were working double shifts which consisted of 13 hours through the night, and others were working 11-hour daytime shifts. Serco acknowledged that it had underestimated demand over the Easter weekend, and that it was unable to cover some GP shifts over the May Bank Holiday, meaning people received a slower service.
:: Supporting staff - the report concluded that not all staff received appropriate training and professional development. A quarter of staff had not completed mandatory training. Although Serco said that all their GPs received formal clinical supervision, not all staff had received regular appraisals.
:: Assessing and monitoring the quality of service provision - inspectors found that the provider did not have an effective system to assess and monitor the quality of service that people receive.
Ian Biggs, deputy director of CQC in the south, said: "At least half a million people depend on this service, so it is vital that it is properly staffed with properly qualified GPs who are available when people need them.
"At times, Serco has not had enough doctors on duty and it is hardly surprising that people have complained. Asking GPs and their drivers to work such long hours should be a last resort.
"However we note that the service has already recruited more GPs and has introduced a new triage system to cope more effectively with calls."
He added: "We now expect Serco to address these issues. The company has 14 days to tell us what they will do to ensure compliance. My inspectors will return at some time in the near future to check that the required improvements have been made and we will report back on our findings."