Obese men have a greater risk of developing one of the most aggressive and life-threatening forms of prostate cancer, scientists warn today.

Researchers in Sweden found that while obese men have an overall lower risk of getting prostate cancer, those who do are more likely to develop a severe form of the disease that quickly spreads around the body and is more likely to kill them.

The finding emerged from a study of 784 men, half of whom were being treated for prostate cancer. Those who were insulin-resistant, a condition closely linked to obesity, were at greatest risk of aggressive prostate cancer, the scientists said.

Obesity plays a role in around 4% of cancers, including breast and womb cancer, and is thought to increase the risk of others, such as bowel and kidney cancer. In most cases, the increased risk is associated with changes in hormones that are released from fatty tissue.

"Possibly, low levels of testosterone in obese men may explain why they are less likely to develop prostate cancer in the first place. But they may be at higher risk of a more aggressive form of cancer that is less reliant on testosterone," said Par Stattin, who led the study at Umea University.

Prostate cancer is the most common cancer in British men, with 31,900 new cases diagnosed each year. Men have a one in 14 chance of being diagnosed with it.

The majority of prostate cancer is slow growing and many men are unaware that they have the disease. The cancer can be treated by removing the prostate gland or using combinations of chemo, radio and hormone therapy.

The early signs of prostate cancer are caused by the gland enlarging. People with the disease may notice blood in their urine and often have to rush to the toilet.

Last year, Cancer Research UK warned that Britain's soaring obesity rates were likely to trigger a wave of cancers, with as many as 12,000 new cases linked to being overweight. After smoking, obesity is considered one of the most preventable causes of cancer.

Projections by the Department of Health foresee the number of obese people in Britain rising to 27.6 million in 2010, a 14% increase on 2003 figures.

According to research in the US, a body mass index of between 30 and 35, the range classified as obese, increases the risk of dying from cancer by one third.

Greg Martin of the World Cancer Research Fund, which backed the research, added: "It is important to remember that being overweight significantly increases your risk of developing a number of different cancers."