Seriously overweight people may find that their premiums could rise 50% but this could increase to 400% if they are also in other high-risk groups.

Body mass index (BMI) is used to calculate those that would pay higher premiums.

Russ Whitworth, director of underwriting and claims at Legal and General, said: “Although it is not an exact science we find that BMI is the best indicator of the risk of being overweight, so it pays to stay in shape.”

Legal and General figures show that those with a BMI of over 30 could pay a higher premium. Customer of Norwich Union would find they face a higher cost if their BMI exceeds 35. Provident have a BMI of 33 as their limit.

An Association of British Insurers spokesperson, said: “If you are obese, you are at greater risk of contracting certain diseases. It is just the same as increasing the premium for a smoker or somebody with previous medical conditions.”