Serco, the leading private contractor to the government, has admitted that it presented false data to the NHS 252 times on the performance of its out-of-hours GP service in Cornwall.

The NHS Cornwall primary care trust (PCT) asked the company to audit itself following an investigation by the Guardian in May, in which several whistleblowers alleged the company was repeatedly so understaffed as to be unsafe and claimed that managers manipulated results when it failed to meet targets.

Serco and the PCT revealed the admission in separate statements on Thursday when a report for the PCT board published online highlighted this and a raft of other concerns about the privatised contract and how it was being run.

The Guardian can reveal that the chair of the powerful parliamentary public accounts committee, the Labour MP Margaret Hodge, has asked the National Audit Office to investigate the case.

The former Tory health secretary and chair of the health select committee, Stephen Dorrell, has also condemned the company's actions. He said the revelation raised wide-ranging concerns about oversight of contracting services which went to the core of maintaining quality in the NHS and should prompt a full review of the Serco contract period.

"To falsify returns once is once too many – to falsify 252 times represents a pattern of behaviour which should lead to a full review," Dorrell said.

The PCT has not yet independently checked the results of the Serco audit, which only covered the first six months of this year when the company had already been told it was under scrutiny. The health regulator the Care Quality Commission reported that staff said that the data manipulation went back four years or more.

The PCT said the Serco service was "fundamentally safe and effective" but had faced "clinical staffing challenges and issues with data handling".

The PCT's chief executive, Steve Moore, told the Guardian that Serco's audit had looked at 107,000 records of patient calls and how they were handled between January and June 2012, and found the 252 unauthorised, unjustified, changes.

"In reviewing the impact, the PCT is clear that the changes were minimal and short-lived," he said. He did not believe that Serco had benefited from altering the data, although the PCT's review was not complete.

Hodge said it was vital that as a growing number of private firms held contracts for delivering public services and took the taxpayer pound they were open to scrutiny.

She said: "If there is any fraud it brings into question whether the company is fit and proper to provide public services. A fraud is a fraud is a fraud. It doesn't matter what the scale is."

In the wake of the Guardian investigation and a damning report from the health watchdog in July, the PCT also commissioned the Department of Health's former primary care tsar, David Colin-Thomé, to conduct an urgent review of the safety and quality of the Cornwall out-of-hours service.

The CQC had found that the Serco service was failing to meet four key legal requirements to provide enough staff and to ensure that its monitoring of its performance was accurate.

Colin-Thomé said he had not found evidence that the current service was, or had been, systematically clinically unsafe, but with "an important caveat: until significant problems are all rectified I cannot say with certainty that the service will remain safe".

Serco's failure to fill shifts for clinical staff remains a serious concern.

The PCT's report also reveals that Serco failed in the last two months to meet targets on passing calls involving an immediately life-threatening problem to the ambulance service within the stipulated three minutes; failed to meet targets for clinical assessment of urgent calls, and missed targets on the length of time it took for people to get through to the service.

While a review found patient satisfaction was high, a survey of local GPs whose patients used the service raised serious concerns about staffing levels, inappropriate assessment of cases and poor sharing of information.

The West Cornwall MP and health select committee member, Andrew George, whom the PCT criticised for blowing the whistle on Serco, to the CQC, said: "Data was altered in a period when Serco knew that it was already under heavy scrutiny and was no doubt trying to clean up its act.

"It was the period before this when I fear such data manipulation would have been worse. This also raises wider concerns about opening up even more of the NHS to private companies who, it seems, will go to any lengths to win and retain contracts."

A Serco statement said the majority of data it examined in its internal audit had been recorded correctly but added that it had said sorry to the PCT for the 252 cases where false data had been reported.

The managing director of Serco's healthcare business, Paul Forden, said: "We apologise that this has happened. We have undertaken this very detailed review and worked openly and transparently with the PCT.

"However we are not in any way complacent and we recognise that we have a number of improvements to implement following the PCT, the CQC and Dr Colin-Thomé reports. We are continuing to put these changes in place."