The disease - the number one killer among women aged 35-54 years - could be triggered early in life by an unhealthy diet of fast food, Professor Paul Kleihues has warned.

Professor Kleihues said parents should avoid high fat, low fibre diets full of processed food, dairy products and meat.

He warned: "Thirty per cent of tumours in breast, prostate and colon cancer are associated with nutrition.

"There is increasing evidence linking breast cancer to dietary habits in the first ten years."

The excess fat carried by overweight girls may be the key to developing breast cancer.

When they reach puberty, the female oestrogen hormones, which help trigger most breast cancers, are stored in the fat instead of being used or discarded.

Eating too much

Poor diet could cause cancer later

Professor Kleihues, director of the WHO International Agency for Research on Cancer, said children in western countries were putting themselves at risk by eating too much.

He said: "The problem is that we eat too much and the food is too rich in fat. A little change in diet is not enough."

He said 35-45% of calories consumed are now from fat. To cut the risk of some cancers this should be reduced by at least ten per cent.

Professor Gordon McVie, director of the Cancer Research Campaign, said the sooner children were encouraged to eat healthily the better.

He said: "There is a lot of circumstantial evidence to support this view. In fact any change in diet right through through puberty will still have an effect.

"There is no doubt that puberty is a critical time in breast development, a lot of things can go wrong in breast growth patterns at that time which can cause problems in later life."

Professor McVie said the impact of a healthy diet had been illustrated by the relative lack of breast cancer among women who were young girls during the second world war.