MPs have launched a scathing attack on a "substandard" out-of-hours GP service run by a private company.
Serco, which provides the service in Cornwall, came under fire last year after a number of whistleblowers came forward with worries about staffing levels.
Concerns were also raised that data had been altered to make the company's performance look better than it was.
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 local health care trust was overstated in seven instances.
But the Public Accounts Committee (PAC) said that it was "disgraceful" that the public only found out about troubles with the service through whistleblowers. They criticised the company for having a "bullying culture" which inhibited workers from coming forward with concerns.
In a new report, the committee said that the service provided by the company fell short of national standards.
"The quality of the service being provided by Serco in Cornwall is not good enough," it states. "People rely on out-of-hours services when they need urgent medical care in the evenings, at weekends and on bank holidays when regular GP surgeries are shut.
"Serco has struggled to ensure enough staff are available to fill all its clinic and car shifts, although it has increased staffing levels in recent months. It has consistently failed to meet the national quality requirements relating to the responsiveness of out-of-hours services and performance is still falling short."
The report also criticised the local health trust responsible for commissioning services in the region for being "ineffective" in managing Serco's performance and for paying performance-related bonuses to the company.
MP Margaret Hodge, chair of the committee, said: "It is disgraceful that the public had to rely on whistleblowers to find out that the out-of-hours GP service in Cornwall, provided by private contractor Serco, was short-staffed and substandard, and that service data was being manipulated, making the company's performance look better than it was."