Soaring obesity rates have saddled Britain with a "cancer timebomb", an expert has warned.

Urgent action is needed to avert a massive increase in the number of cancer cases, according to Dr Greg Martin.

Research shows that up to 40 per cent of cancers can be prevented through a healthy diet and regular exercise, he said.

Dr Martin, science and research manager at the World Cancer Research Fund (WCRF), said: "There has been plenty of attention recently on the problems of obesity, but a lot of people still do not realise how closely obesity is linked to cancer.

"People getting fatter will lead to an increase in the number of cancer cases as surely as night follows day. You also have to add into the mix that cancer is largely an older person's disease, and the UK has an ageing population.

"So,,if you have an ageing population that is getting more obese, there could be really serious consequences in terms of the number of people getting cancer if people do not act now. It's a cancer timebomb."

He was speaking at the start of Cancer Prevention Week, organised by the WCRF.

In 2003, an estimated 4.3 million British men were obese - defined as having a Body Mass Index (BMI) of 30 or above.

BMI is calculated by dividing weight in kilograms by height in metres squared.

By 2010, that figure is expected to have risen to 6.6 million.

"It's a huge chunk of the population," said Dr Martin. "Each of these 6.6 million people are going to have an increased risk of cancer, but this is an avoidable risk factor.

"This kind of increase cannot be explained by anything genetic. It's almost certainly a function of behaviour. It's something we can do something about."

Obesity is particularly linked to hormone-sensitive cancers such as breast cancer and endometrial cancer, cancer of the womb lining. It is also associated with oesophagus and bowel cancer, which can affect both women and men.

Dr Martin said he expected cancer rates to follow the pattern of obesity, which was spreading from one generation to another like an infectious disease. "When you look at a time series of maps it's really like an epidemic, a virus spreading," he said.

In poorer countries, people are now suffering from chronic diseases as well as ancient infections as a result of embracing Western diets and lifestyles.

"The frustrating thing is that people can make simple changes to their lifestyle to reduce their chances of getting cancer," said Dr Martin. "But ultimately each person has to make that commitment, and at the moment too many people are dying because they are not taking action.

"It really is as simple as eating more healthily, making sure you portion size is not too big, and taking regular exercise. When you think about what a devastating disease cancer is, it really is worth taking these simple steps."

As part of Cancer Prevention Week, the WCRF is organising Fruity Friday.

The aim is to increase people's awareness of the importance of eating fruit and vegetables.

Organisations across the UK will be getting involved, with companies laying on fruity breakfasts for employees, and schoolchildren taking part in fruit-orientated events.