Some patients in Hertfordshire are to be told they will have to lose weight before they can have certain types of surgery.
The new guidance has been issued to GPs by NHS Hertfordshire this week.
It is aimed at patients with a body mass index (BMI) of more than 30 who need non-urgent hip or knee joint replacement surgery.
Smokers will also be asked to meet with an NHS stop smoking specialist before they can be added to the waiting list.
Dr Tony Kostick, chair of the East and North Herts GP Consortium, said that the new guidelines were issued after a recommendation by the Clinical Executive Committee of the Trust.
"It followed a pretty extensive review of all the clinical evidence by our public health colleagues.
"They've said that patients have less post-operative complications and improved clinical outcomes if they lose weight and give up smoking," he said.
The guidelines state that patients with a body mass index (BMI) of more than 30 will need to reduce their body weight by 10% or reduce their BMI to below 35, whichever is the greater, before they can be added to the waiting list.
"We're not doing this out of malice or spite.
"It's for no other reason other than it's good clinical practice. People will do better from their surgery and gain more benefit from their surgery if they lose weight," Dr Kostick said.
He said smokers will not be denied access to surgery, but they will be asked to make an "informed decision".
"What we're saying is that we would like [patients] to go to a smoking cessation service and discuss the risks with them and then they'll be considered for surgery."
The Hertfordshire guidelines are different from surrounding counties.
NHS Buckinghamshire have said that they are not going to introduce these rules. They advise patients to stop smoking or lose weight, but they don't enforce it.
NHS Bedfordshire said it was also only advising patients for now.