LONDON - Smoking is well known as a risk factor for type 2 diabetes, but scientists said on Monday that quitting the habit can raise the risk even more in the short term.

A study by U.S. researchers found that people who stop smoking have a 70 percent increased risk of developing type 2 diabetes in the first six years without cigarettes as compared to people who never smoked.

The researchers said they suspected the increased diabetes risk comes from extra weight gain common in people who quit.

But they said no one should use their findings as an excuse to continue smoking -- a habit which can also cause lung disease, heart disease, strokes and many types of cancer.

"The message is: Don't even start to smoke," said Hsin-Chieh Yeh of the Johns Hopkins University School of Medicine in the United States, who led the study.

"If you smoke, give it up. That's the right thing to do. But people have to also watch their weight," she added.

Type 2 diabetes -- often called adult-onset diabetes -- is a common disease that interferes with the body's ability to properly use sugar and insulin, a substance produced by the pancreas which normally lowers blood sugar after eating.

Overweight people and those with a family history of the disease have an increased risk of developing it, as do smokers.

Diabetes is reaching epidemic levels, with an estimated 180 million people suffering from it around the world.

Diabetes cases are forecast to triple in the United States in the next 25 years to 44 million with the costs of caring for them rising to $336 billion a year.

Yeh's study, published in the Annals of Internal Medicine journal, looked at almost 11,000 middle-aged adults who did not yet have diabetes from 1987 to 1989. The patients were followed for up to 17 years and data about diabetes status, glucose levels, weight and more were collected at regular intervals.

The researchers found that people who quit smoking had a 70 percent increased risk of developing type 2 diabetes in the first six years after stopping compared to people who never smoked. The risks were highest in the first three years, and returned to normal after 10 years.

Among those who did not stop smoking the risk was lower, but the chance of developing diabetes was still 30 percent higher compared with those who never smoked.

Tobacco is the leading preventable cause of death in the world, killing more than 5 million people a year. A report by the World Lung Foundation last August said smoking could kill a billion people this century if trends hold.