Britain's largest private health insurer is embroiled in a severe row with some of the country's top surgeons after accusing them of performing operations that earn them £600 a time on patients without a good medical reason.

Bupa, which insures 3 million Britons, claims orthopaedic surgeons have carried out unnecessary knee arthroscopies on some of their clients.

The company alleges that unnamed specialist surgeons have defied standard clinical practice and official advice by subjecting some patients to the procedure.

Bupa members are more than twice as likely as NHS patients to have the operation, while some surgeons are three times more likely than others to recommend that patients they examine undergo it, the firm claims.

But surgeons' leaders have responded by alleging that Bupa is damaging their reputations with an unjustified "slur", undermining patients' trust in their doctor, and trying to save money by denying people in pain an operation needed to restore their quality of life.

Dr Annabel Bentley, the medical director of Bupa's UK health and wellbeing division, has sparked a major dispute by drawing attention to what she says are big differences in NHS and private patients' chances of having the operation.

In a letter to the British Orthopaedic Association (BOA), which represents surgeons who perform joint operations, she wrote: "We have concerns about current clinical practice in knee arthroscopy because the rate of surgery performed on our insured members is substantially higher than the NHS rate. We have also identified specific cases among Bupa members by some consultants where treatment does not appear to be in line with published evidence-based guidelines."

Speaking to the Guardian, Bentley claimed: "We found the rate of knee arthroscopy in our members is more than double the rate in NHS patients [and that] some surgeons are three times more likely than others to perform a knee arthroscopy on our members."

While some variation may be due to clinical reasons, these big differences in treatment rates are unexplained, she added.

Surgeons' organisations have criticised Bentley for making such claims without referring doctors Bupa believes are over-operating to the General Medical Council for misconduct, or publishing the evidence it says it has of the unjustifiably wide variation.

The BOA, British Association for Surgery of the Knee (Bask) and the Federation of Independent Practitioner Organisations (Fipo) are opposed to a new pre-surgery check Bupa has introduced in a bid to reduce unnecessary knee arthroscopies.

Bupa wants all surgeons to fill in a form giving details of the clinical indications for surgery before it approves payment.

In a letter to the Bupa's board, the three organisations voiced "concern at what we believe is a threat to clinical governance, patient care and consultant independence" because "this new approval process for surgery involves non-medical Bupa staff checking a surgeon's decision against undisclosed guidelines".

The BOA has urged the 4,000 surgeons it represents to boycott the forms, which, it says, undermine surgeons' clinical judgment.

While some are completing them, others are refusing to do so, and some are being threatened with what the BOA's president, Peter Kay, describes as "blacklisting" – no longer being allowed to treat Bupa patients.

The letter calls Bupa's actions "indefensible". It says: "At the heart of what is presented to us as a funding or cost control measure it is alleged by the doctors in the Bupa health and wellbeing division that some surgeons are carrying out inappropriate surgery. However, no specific evidence of this has been provided."

It also says Bupa subscribers will cancel their policies if they are denied the chance to use the surgeon who examined them.

Bentley denied Bupa's move was intended to save money. "We want to ensure we pay for best practice, not poor practice," she said.

Health insurers are under pressure as the number of policyholders has dropped sharply in the past two years as a result of the financial squeeze.

Fergus Craig, the commercial director of AXA PPP, another big health insurer, indicated recently that it believes some surgeons are over-treating for personal gain.

He said: "The rate of intervention in some areas [of medical practice] is higher than it is in others. This concerns me. There is a clear incentive for consultants to do things that may be of marginal medical benefit and significant benefit in terms of bolstering their income."

Bentley said "it's a possibility" that some surgeons are performing unnecessary operations. The big difference in treatment rates may be due to some under-treating but others over-treating, she said.

She said she did not know if individual surgeons were doing too many operations in order to make money, but "had a duty to act when we see this issue, and we see this issue with knee arthroscopy".

Bask's president, Tim Wilton, said patients denied their choice of specialist knee surgeon were being offered alternative surgeons with expertise in back, shoulder or feet operations, a development which could affect patients' welfare.

But Bentley added that Bupa was simply repeating an exercise it did several years ago when, on examining high rates of hysterectomy among its patients, it found some women were having a healthy womb removed by surgeons.

Patient who paid

Alison Twigley, a consultant anaesthetist, was denied her chosen surgeon for a knee operation in June when Bupa refused to fund it because he refused to fill in a form that surgeons claim undermines their clinical judgment. She still used the surgeon but paid for the operation herself.

"I hurt my left knee last December after dragging a heavy new table around when I had a new kitchen put in. Bupa paid for an MRI scan, which showed that I'd torn my cartilage.

"My knee swelled up due to fluid on it. It was fairly stiff, I had some pain and I couldn't flex it much. I went on a skiing holiday but couldn't ski or do much physical exercise to keep fit as I usually do.

"I was very reluctant to have an operation. I wanted to see if the knee would mend on its own. But Prof Tim Briggs, one of the country's leading orthopaedic surgeons, told me it would probably need something doing to it surgically.

"In April I agreed to have an arthroscopy and have the cartilage trimmed as the knee was obviously getting no better.

"But in late May the Spire hospital in Bushey in Hertfordshire where I was due to have the operation on 17 June contacted me to say there was a problem with my Bupa insurance.

"Bupa told me it couldn't fund the operation because Prof Briggs hadn't signed their its special new form giving the grounds for the arthroscopy. I challenged them but they were adamant.

"Prof Briggs told me he hadn't signed it because it was undermining his professional judgment.

"He said: 'Why should an insurance company decide whether a patient had or didn't have an arthroscopy?'

"I agreed with him. It's wrong for insurance company employees to judge whether a consultant orthopaedic surgeon should be doing one operation or another because they won't have the clinical judgment. They must be trying to save money.

"When I spoke to Bupa it said: 'There's no reason to have it done with Prof Briggs. We'll give you the name of a surgeon who is happy to sign the form.'

"That really incensed me. I said I wanted a surgeon I trusted and knew would do a good job. I ended up paying for the operation myself.

"I thought Bupa's action was breach of contract. What's the point of being insured with somebody if when you need something done it won't pay for it?

"I was paying them it £127 a month. But I've now cancelled my policy, so I will get the cost of the operation back that way."

Briggs confirmed that Twigley had wanted to see him and only him.

"I refused to sign the Bupa form as a matter of principle," he said. "Bupa's new policy is limiting people's choice of surgeon because a lot of consultants are refusing to sign these forms."