A Cornish out-of-hours GP service has been severely criticised by the Care Quality Commission (CQC).
In the CQC report, Serco has been told it has not met four of the essential standards of quality and safety.
Serco has been given 14 days to provide a report setting out how it will meet the necessary improvements.
Dr Jeremy Mawer, Medical Director for the out-of-hours service said Serco was "committed to providing high quality, accessible services".
Commission inspectors found that people who use the safeguarding people from abuse service "were not protected from the risk of abuse, because not all staff were trained in the safeguarding of vulnerable adults and children".
They also found that 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, the report stated "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.
The CQC said providers had "a legal responsibility to make sure they are meeting all the essential standards".
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 that people received a slower service.
The report also revealed that a quarter of staff had not completed mandatory training. Although Serco said all their GPs received formal clinical supervision, not all staff had received regular appraisals.
Inspectors also noted that "the provider did not have an effective system to assess and monitor the quality of service that people receive".
Proper staffing 'vital'
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.
"However we note that the service has already recruited more GPs and has introduced a new triage system to cope more effectively with calls."
Paul Forden, managing director of Serco's Clinical Services, said: "We acknowledge that at the time of the CQC visit earlier this year, we did not meet all aspects of four standards against which we were being audited.
"One area was found to be of 'minor' concern and three to be of 'moderate' concern.
"We can confirm that we have already implemented actions to ensure that three of the four areas have made progress and we consider that we have achieved the required standards. On the fourth recommendation on training we are 92% compliant today and will fully meet the requirement within the next month."