Suspected international PMI fraud of "considerable" size is currently under investigation after increasing activity in recent years, Axa PPP has reported.
According to the healthcare insurer, the biggest changes in PMI fraud are occurring internationally where insurers are more vulnerable.
Dr Simon Peck, head of investigations at Axa PPP healthcare, said: "We are fortunate in this country that the majority of health providers are trustworthy and work to the highest standards. However there are some problems.
"The biggest changes we have seen are in the international arena where insurers are more vulnerable to fraud. There is growing cooperation between insurers and this is leading to us discovering more problems but more importantly developing new solutions."
Peck said cooperation between insurers in the UK and in the USA was "unprecedented" in tackling fraudulent activity.
He added that PMI fraud was not a driver of inflation but was a cost to the industry and added to premiums.
Dr Annabel Bentley, medical director at Bupa Health Funding, said of PMI in the UK: "In the past, the emphasis of fraudulent PMI claims was on inappropriate billing from providers.
"However, system enhancements over the years have allowed us to detect and eradicate instances of mis-billing by providers, in particular unbundling of procedure codes."
She added the cost of fraudulent claims could potentially impact on health insurance premiums if it went undetected.