Its author, Dr David Colin-Thomè, was called in by the NHS Cornwall and Isles of Scilly, which commissions the service from private company Serco, after concerns were raised by the Care Quality Commission this summer.
After a series of surprise visits, the commission ruled that the firm was failing to meet four essential targets and said it was "possible" that its performance may have been "overstated" because managers routinely altered negative data.
Dr Colin-Thomè's report was published yesterday, ahead of being presented to next Tuesday's meeting of the health service board. He said he had found no evidence "that the current out-of-hours service is or has been systematically clinically unsafe".
But he warned: "However an important caveat – until significant problems are all rectified, I cannot say with certainty that the service will remain safe.
"The main and continuing problem, despite improvement, is the insufficient numbers of general medical practitioners to man the service."
Steve Moore, chief executive of NHS Cornwall and Isles of Scilly, said he was "disappointed" that data used to measure performance was "inaccurate" although the numbers were "small".
He added: "I am clear that Serco did not gain from these actions and they have issued a full apology to us."
