The NHS fraud squad is investigating GPs in England amid suspicions they are claiming for non-existent patients.
Doctors get an average of £150 a year for each patient on their list, but records show there were 3.6 million more patients in the system last year than there were people in England.
The discrepancy prompted NHS England to employ a company to start chasing up these so-called ghost patients.
The NHS Counter Fraud Authority is now launching its own investigation.
Doctors' leaders have always insisted the issue of ghost patients most often has an innocent explanation, such as instances where patients have died or moved without the knowledge of their GP.
It is understood the list-cleaning exercise, being carried out for NHS England by the business services company Capita has started to see a reduction in the numbers being claimed for.
It has focused on patients who have not visited their doctor for five years.
Attempts have been made to contact those patients and where they have not been found they have been deregistered from the practice.
But NHS fraud investigators have been carrying out some sample testing of transactions, which the BBC understands has identified some "anomalies" that have raised suspicions.
The fraud team will now carry out a full analysis of records held by NHS England and the NHS Business Services Authority to see if doctors have been fraudulently claiming for patients.
Investigators believe the funding system for registered patients is particularly vulnerable to fraud.
The average GP has around 1,700 patients on their list so the payments make up a significant chunk of their income.
The fraud team have estimated that up to £88m may be being incorrectly claimed for - around 1% of the GP budget.
NHS fraud chief Susan Frith said the focus on GPs was just one of a number of priorities for the coming year.
"By preventing fraud, by identifying it and tackling it effectively where it occurs, and by seeking to recover moneys lost to fraud we can ensure that precious NHS funds are used for their intended purpose of patient care."
Dr Richard Vautrey, of the British Medical Association, said it would be wrong to jump to conclusions.
"Some of these will be people that have recently died, or left the country, others may be homeless or simply unaccounted for in government statistics, and we would be concerned at any suggestion that any discrepancies are down to wilful deception by hard-working GPs."