The Care Quality Commission (CQC) has raised concerns over the state of out-of-hours GP services, following an investigation into a pensioner's death.
The report concluded there may be a "nationwide problem" with private healthcare companies working under NHS contracts.
It follows the death of David Gray in February 2008, who was accidentally killed by a German doctor who admitted to being exhausted before starting the out-of-hours shift for Cambridgeshire health trust.
Dr Daniel Ubani gave Mr Gray 10 times the normal dose of diamorphine that was required.
The CQC report looked at the Take Care Now company which Dr Ubani worked for. It currently has contracts with five primary care trusts across England. These are with Worcestershire, Cambridgeshire, Suffolk, Great Yarmouth and Waveney and South West Essex PCTs.
The report concluded that PCTs should scrutinise private companies more closely and consider the tasks they are given to carry out, including call handling, the number of unfilled shifts, and the proportion of shifts covered by non local doctors. It also raised concerns over the decisions made by non-clinical staff.
Chief executive of the CQC, Cynthia Bower, said the report on Take Care Now was only the tip of the iceberg.
"If their monitoring is not robust enough, they may not be in a position to spot early indications of potential problems or poor care."
She added: "We believe this may point towards a national problem."
Ms Bower urged all PCTs to conduct a review of their out-of-hours services.