Concerns have been raised over the provision of out-of-hours GP care by private medical companies following a report into the death of a man who was given a lethal dose of medicine.
The Care Quality Commission (CQC) said a "nationwide problem" may exist, and urged those in charge of organising contracts between NHS trusts and the private companies to review the system.
A report into the death of 70-year-old David Gray showed that he was given a dose of diamorphine 10 times the amount that was required by a German doctor.
Dr Daniel Ubani later admitted he had been exhausted when he started the out-of-hours shift for Cambridgeshire health trust in February 2008.
The report examines the work of a private firm, Take Care Now, which employed Dr Ubani and which currently has contracts with five primary care trusts (PCTs) in Worcestershire, Cambridgeshire, Suffolk, Great Yarmouth and Waveney and South West Essex.
The report said: "All PCTs should scrutinise out-of-hours services more closely.
"They should look in detail at the services that they commission, including the efficiency of call handling and triage, the number of unfilled shifts, the proportion of shifts covered by non-local doctors, the induction and training those doctors receive, and the quality of the decisions made by clinical staff."
Cynthia Bower, CQC's Chief Executive, said current trust monitoring of Take Care Now's services was "only scratching the surface."
The CQC has shared its findings with the Department of Health, which is writing to PCTs across England asking them to make it a priority to review their monitoring arrangements for out-of-hours services.
Health Minister Mike O'Brien said: "Patient safety is paramount and PCTs have a clear legal responsibility to provide safe, high quality out-of-hours care, and are required to have in place robust performance management arrangements to ensure their out-of-hours services are delivering against contractual requirements."