Some obese people are eating more fatty foods to qualify for weight reduction surgery on the NHS, a health charity claimed yesterday. Patients who are severely or morbidly obese - but not quite big enough for an operation - are trying to put on weight, according to the British Obesity Surgery Patient Association (Bospa).

Health trusts' criteria for surgery differ, leaving obese people confused and angry, says Bospa. Obesity is measured according to the body mass index (BMI), which is calculated by dividing the patient's weight in kilos by their height in metres squared. A score of 35-40 is regarded as clinically obese, while 40-45 is morbidly obese.

Guidelines suggest surgery such as gastric bands, which squeeze the stomach, should be available for people who have a BMI of 35 and related medical conditions. But some trusts have put the lower limit at 40 or 45. Bospa said patients not entitled to surgery on the NHS claimed people in other parts of the country were entitled. Up to 10 had said they were going to put on weight to qualify and accused the trusts of putting the limit up to save money.

Bospa trustee Chrissie Palmer said: "This is yet another process that has been turned into a postcode lottery. Most of the people we deal with who were thinking of surgery are now completely bemused. Do they carry on fighting a losing battle or put on a few extra pounds to make the grade?"

Sig Lonegren, of Glastonbury in Somerset, said he was not entitled to surgery even though his BMI is 37.3. He said he had tried 20 diets but had still put on weight and now weighed nearly 20 stone. Mr Lonegren, 67, who has diabetes, high blood pressure and high cholesterol, said: "The situation is absurd. Instead of surgery and rehabilitation my only option is McDonald's and ice cream. It's grossly unfair and incredibly short-sighted. There are hundreds of other people like me who can't afford private surgery, so the message seems to be pay up or pig out."

Mr Longren's health trust, Somerset PCT, said that overeating to hit a target was dangerous and misguided.

It denied it had put the limit up to 45 to save money, saying it was only one of a number of criteria considered when looking at the option of surgery.