Patients with type 2 diabetes who are obese could be wiping eight years off their life expectancy, according to research published today.

A study of 44,000 people with so-called "adult onset" diabetes found that having a high body mass index was a leading cause of premature death.

The research, conducted by the University of Surrey, the Royal Free Hospital and University College London, was presented at the annual medical conference of charity Diabetes UK in Birmingham.

The number of people with type 2 diabetes has increased as the obesity epidemic has spread across the UK. Doctors are also increasingly reporting the condition appearing in children, when previously type 2 diabetes was only seen in adults.

Diabetes UK called on overweight diabetics to seek help with their diet in a bid to cut early deaths. The charity's chief executive, Benet Middleton, said: "80% of people are overweight when diagnosed with Type 2 diabetes so this research is very worrying.

"It is essential that people with diabetes manage their weight so they can help to reduce their risk of serious complications such as heart attack and stroke. Obesity is taking years off people's lives."

Concern has been growing about the increasing numbers of adults and children classified as overweight or obese. Diabetes UK said people with diabetes already had a higher risk of complications and premature death, which was made even worse by obesity.

The charity said people with diabetes who were trying to lose weight should be supported by their diabetes care team and have access to a state-registered dietician.

In a separate study, also presented at the conference, it emerged that stem cell research could offer hope for a cure for patients with type 1 diabetes.

Researchers from King's College London found that it was possible to switch on some of the genes in human embryonic stem cells that had important functions in the hormone-producing cells in the pancreas.

But presenting their early results to the conference, the researchers warned that this was only the first step towards a therapy. They said the next challenge would be to re-create the body's natural responses to release insulin and glucagon in response to blood glucose levels.

Eleanor Kennedy, research director at Diabetes UK, said the research was very exciting. "It gives a strong indication that stem cells may hold the promise of a cure for Type 1 diabetes. We'll be watching progress in this area closely."

Last year, researchers in the United States found that stem cells from bone marrow could be converted into insulin-producing beta cells in the pancreas to replace those damaged by diabetes, raising the possibility of future treatments.