Whistleblowers were "significant" in uncovering problems at a privatised out-of-hours GP service where data was falsified to disguise understaffing, a financial watchdog found.
The National Audit Office (NOA) investigated Serco's provision of Cornwall's out-of-hours GP service, which came under fire last year after whistleblowers raised concerns that performance data was being altered.
Serco's own investigation found that two staff made 252 "unauthorised changes" to the data and, as a result, the performance reported to the primary care trust was overstated in seven instances.
Health regulator the Care Quality Commission subsequently said that Serco had not met four of the essential standards of quality and safety.
The NAO report stated that last year Serco, given £32 million to provide the out-of-hours contract, had been unable to fill shifts with appropriately qualified staff.
Whistleblowers played a "significant role" in highlighting the problems and the NAO recommended that the Department of Health does more to protect those coming forward.
"This is an issue that is not confined to the out-of-hours service in Cornwall," the report stressed.
"The government has previously recognised that, although whistleblowers are legally protected, practice on the ground in the NHS has not always been effective."
Dr Louis Warren, who manages the Serco service in Cornwall, said: "While whistleblowers highlighted concerns last year, I am confident that these issues have been addressed."