Overweight patients have been told to shift excess pounds before they are allowed operations such as hip and knee replacement, tonsil removal or gall bladder surgery.
It is hoped the new policy will result in patients being in a better state to recover following non-urgent procedures.
Today the Herts Valleys Clinical Commissioning Group held a conference about the scheme, which has been officially running since January 1.
Dr Nicolas Small, chairman of the group, said: “GPs in the area will not list patients for routine surgery until they have lost weight. There is a range of support, some from the GP surgery, or they can be sent to groups such as Rosemary Conley or Weight Watchers.”
Nationally, one in four adults is obese, and one in eight is severely overweight. Severely obese people can expect to die an average of eleven years earlier.
Now anybody with a body mass index of more than 35 will have to lost weight before they are eligible for non-urgent surgery.
A person of five feet and ten inches would need to be 17 stone and ten pounds to have a BMI of 35. A "healthy" BMI is between 20 and 25.
Dr Small said: “Surgery is more effective if people lose weight first and then surgery might not be needed at all. This is a trigger to help people lose weight.
“There is always a risk when patients are given an anaesthetic but there is strong clinical evidence that this risk is significantly higher when they are overweight.”
The group launched a similar scheme last year when overweight people wanting hip or knee surgery were told to lose weight before being listed.
An obese person is 15 times more likely to need a hip or knee operation and there were 5,000 of these replacements in West Hertfordshire last year.
Smokers will also have to agree to meet with a cessation advisor before being listed for surgery.
Rachel Joyce, consultant in public health, said: “Smoking and obesity are responsible for the vast majority of deaths in Hertfordshire.”
The Herts Valleys Clinical Commissioning Group, a consortium of GPs, will take over responsibility for buying health services when the country's Primary Care Trusts are scrapped in 2013. It covers about half of the county and will be responsible for Watford, Three Rivers and Hertsmere, as well as St Albans, Harpenden and Dacorum.
Dr Sheila Borkett-Jones, board member for Watford and Three Rivers, said: “It's a free country, but it's our job to inform people about living healthily.”
There is currently no plan to introduce a similar scheme for seriously underweight patients who are waiting for routine surgery.
Dr Borkett-Jones added: “We don't suffer from an epidemic of people who are underweight.”