More needs to be done to protect whistleblowers in the NHS, the government has been warned.
The National Audit Office said NHS managers should be held to account if whistleblowers suffer reprisals for voicing their concerns.
It told the Department of Health to "take the lead" to make sure that people who come forward with concerns feel protected.
Whistleblowing in the NHS is under the spotlight after chief executive Sir David Nicholson was grilled by MPs about the Mid Staffs scandal.
He was asked about United Lincolnshire Hospitals Trust, which tried to silence a former chief executive from speaking out about patient safety concerns as part of a unfair dismissal case settlement.
Gary Walker was threatened with legal action related to his reported £500,000 payout after breaking his silence to allege he was forced out of his job in 2010 because he put patient safety ahead of Whitehall targets.
The trust is one of 14 being investigated by health chiefs over high mortality rates in the wake of the public inquiry report into serious failures at Mid Staffordshire NHS Foundation Trust that led to hundreds of unnecessary deaths.
The NAO report, which examined concerns over Cornwall's out-of-hours doctors service, said whistleblowers played a significant role in highlighting problems with the service.
"The Department of Health should take the lead in making sure that whistleblowers are, and feel, protected throughout the NHS," it said.
"Whistleblowers are a valuable source of intelligence and should be encouraged to come forward."
Serco, the company that provides the out-of-hours GP service in the region, came under fire last year after a number of whistleblowers came forward with worries about the service.
Concerns were raised that Serco staff were altering data about the performance of the out-of-hours service.
An audit by the firm found that two members of staff made 252 "unauthorised changes" to performance data and, as a result, the performance Serco reported to the primary care trust was overstated in seven instances.
Health minister Lord Howe said: "It is absolutely critical that all staff working within the NHS feel able to speak up and raise concerns and that every organisation providing NHS services takes concerns seriously and acts on them."