Competition Commission statistics - as part of its private healthcare market investigation - have shown consultants do not reference insurer guidelines when making patient fee levels.
The survey of 570 consultants showed a majorit (76%) said they did not know how they set their fees and that they did not belong to any group that set guidelines; only 1% referenced insurer guidelines when setting fee levels.
Of consultants who do not usually set the same fees to all patients 57% said they set different fees for self-pay and insured patients and 55% said there would be varying fees depending on the insurer.
But of total 570 respondents 91% charged the same fees for both insured and self-pay patients.
Only 3% overall set lower fees for self-pay patients than insured with 81% of those doing so for Bupa-insured patients - totalling a negligible 13 consultants - with the same figure for AXA-insured patients.
Brian Walters, Regency Health, said: "The survey indicates that only 39% of consultants charge within all insurers' guidelines, which is a concern. It also suggests that the vast majority of patients pay the shortfall, with only 4% opting for another consultant and 9% falling back on the NHS.
"This is an area where PMI often falls short of consumers' expectations and is something that needs addressing. PruHealth lead the way with their full-refund guarantee, which is based on a sophisticated approach to negotiating with consultants on a case-by-case basis."
There was no agreement in place between 19% of consultants and private insurers to abide by insurer reimbursement when setting fees.
And the average level of top-up that insured clients have to pay, according to the data, is 18% of the fees in cases where the levels are more than their policy covers.
When fee levels are above insurer reimbursement 86% of consultants said the patient paid a top-up to cover the shortfall.
Consultants that negotiated with insurers to pay the amount in full totalled 3% and 9% said patients referred back to the NHS while 55% said they did not set fees above the level of insurer reimbursement.
Charlie MacEwan, director of communications of WPA, said: "We pay consultants what we believe is customary and reasonable. Most charge within those guidelines while others charge more than is customary because they believe they have higher expertise or skills."
WPA said 98.2% of its claims were paid in full and that shortfall was rarely an issue.
MacEwan added: "It is a communication thing. It is about people establishing fees with consultants from the outset."
Financial questionnaires were sent out by the Competition Commission in July 2012 to the larger insurers and the largest hospital operators.
Questionnaires were sent to the 30 largest corporate customers of the five largest private medical insurers and to 100 anaesthetist groups. The CC has also undertaken surveys of general practitioners (GPs), private consultants and patients.
