St Ives Liberal Democrat MP Andrew George referred concerns about private contractor Serco to the Care Quality Commission (CQC).
Its report in July, which followed a series of unannounced inspections, raised concerns over staffing shortages and changes to daily performance reports which could have portrayed the firm in a more favourable light.
Yesterday, a report by Dr David Colin-Thomè, an independent health consultant who was called in to assess Serco's service, was published by NHS Cornwall and Isles of Scilly, which commissions the firm.
"I must say at the outset I did not find any evidence that the current out-of-hours service is or has been systematically clinically unsafe," Dr Colin-Thomè concluded.
"As with most health care organisations, there have been cases of poor care but no systematic unsafe care.
"But until all significant problems and in particular those of GP staffing are rectified, I cannot say with certainty that the service will remain safe."
The conclusions, though, did not satisfy Mr George who said: "The report confirms that managers deliberately altered data.
"The data manipulation would have helped Serco present figures which would have inflated its response time targets.
"Data was altered on 252 occasions between January and June this year – a period when Serco knew that it was already under heavy scrutiny and was no doubt trying to clean up its act.
"It was also the period when I had referred the matter to the CQC and when the CQC commenced and completed its investigation."
The report will be discussed by the board of NHS Cornwall and Isles of Scilly on Tuesday.
Steve Moore, chief executive of NHS Cornwall and Isles of Scilly, said: "I was disappointed to learn that some of the data we received to measure performance was inaccurate although the number of inaccurate records was small.
"I am clear that Serco did not gain from these actions and they have issued a full apology to us.
"I am pleased to say that patient satisfaction with the service remains high and Serco is working constructively with us to address the issues raised by our review."
Paul Forden, managing director of Serco's clinical healthcare business, said it had examined 107,000 records and 252 "found insufficient, inconsistent or conflicting evidence from the various data sources used".
He said: "The report also acknowledges that we have put in place an action plan to address all the issues raised in the earlier CQC report.
"This is making excellent progress and all the required actions will be complete by the end of this month. We are pleased that our service continues to be highly rated in independent patient surveys."
