Surgery to reduce the stomach and induce weight loss could have a significant benefit for hundreds of thousands of diabetes patients, health experts say.

People with diabetes should be offered gastric surgery as a standard treatment option which could help control it for years without medication, the world's leading diabetes groups say.

A joint statement from a 45-strong group said on Tuesday that bariatric, or metabolic, surgery could have a significant benefit for hundreds of thousands of patients worldwide, which they said represented one of the biggest shifts in diabetes treatment guidelines since the advent of insulin.

The new guidelines say surgery to reduce the stomach and induce weight loss should be recommended to treat all diabetes patients whose body mass index (BMI) is 40 or over, regardless of their blood glucose control, as well as those with a BMI of 30 and over whose blood sugar levels are not being controlled by lifestyle changes or medication.

Francesco Rubin, a professor and chair of metabolic and bariatric surgery at King's College London and one of the authors of the new guidelines, said many countries across the world are in the midst of "an epidemic of diabetes".

Rubino said patients should be offered a range of options, including lifestyle changes, medications and surgery.

"For some, surgery may be the best choice," he told a briefing in London.

Type 2 diabetes is characterised by insulin resistance, which many can manage with medication and diet. But the disease is often life-long and a major cause of blindness, kidney failure, heart attacks, stroke and lower limb amputation.

A recent World Health Organisation study found that the number of adults with diabetes has quadrupled in the past four decades to 422 million.

International Diabetes Federation (IDF) estimates that by 2040 this will rise to 642 million.

"Surgery represents a radical departure from conventional approaches to diabetes," Rubino said.

The guidelines, published in the journal Diabetes Care, were endorsed by 45 international organisations, diabetes specialists and researchers, including the IDF, the American Diabetes Association, the Chinese Diabetes Society and Diabetes India.

The guidelines are based on a substantial body of evidence, including 11 randomised trials, showing that in most cases surgery can lead to reductions in blood glucose levels below the Type 2 diabetes diagnosis threshold or to a substantial improvement in blood glucose levels.

In many cases this would lead to patients being able to give up or significantly reduce their diabetes medications.