The ads are designed to 'shock' people

Pictures of a overweight man's rear end are to be plastered on buses with the slogan "Don't look like the back end of a bus".

The initiative, which is to be launched on Monday, is part of a campaign by the World Cancer Research Fund to highlight links between obesity and cancer.

It said the aim was to shock people into thinking about diet and exercise.

"People who aren't yet overweight also need to take notice," the charity's adviser Professor Martin Wiseman said.

In 2002, figures from the National Audit Office estimated that 70% of men and 63% of women were overweight.

Quadrupled

Obesity had trebled in women since 1980 from 8% to 23%, while it had almost quadrupled in men from 6% to 22%.

The Body Mass Index or BMI is used to determine obesity - those with a BMI over 30 are defined as obese.

Prof Wiseman said being overweight was responsible for more than 9,000 cases of cancer every year.

The ads will be appearing on buses on 16 different routes across the capital.