Diets of junk food and fizzy drinks could be leading to a rise in cases of bowel cancer in younger people, a top doctor has warned.

Professor Will Steward, professor of oncology at Leicester Royal Infirmary, spoke out as figures revealed the number of people under 30 with bowel cancer had doubled in the past decade.

He appealed to young people to cut out fast food.

Prof Steward said: "These figures are extremely worrying.

"People under 30 don't expect to get colon cancer. It is far more common in people over 70 and is thought of as an old people's cancer.

"It can't be genetic as the people diagnosed are too young and the rise in numbers is too sudden.

"It is difficult to detect the cause, but it would seem modern lifestyles, with people having high-calorie fizzy drinks and fast food, could be the cause."

Prof Steward, who is also head of the department of cancer studies and molecular medicine at Leicester University, is concerned young people and GPs might not take symptoms seriously.

He said: "GPs won't be expecting people in this age to develop this cancer and nor will the patients, which could lead to a delay in diagnosis.

"The sad thing is you can cure bowel cancer if it is caught early enough but, if you don't, there is only a one in 10 chance you will be alive in five years."

Figures published by the Department of Health showed the number of men under 30 with bowel cancer doubled from 34 in England and Wales in 1997 to 68 in 2006.

The number of women diagnosed with the cancer went from 29 in 1997 to 69 in 2006 – a 138 per cent increase.

In the East Midlands, the number of cases rose from two to four. Two people in Leicestershire have died.

Patients over 40 are entitled to screening for bowel cancer, but it is not routinely available to anyone younger.

Prof Steward said theories being explored were that processed foods and fizzy drinks remained in the bowel longer and had the potential to make cells malignant, causing the cancer to develop.

Hilary Whittaker, chief executive of the charity Beating Bowel Cancer, said: "We have been aware for some time, from the calls and e-mails we get to the charity from young people and their families that there are rising numbers of young people being diagnosed with this disease, but these figures are truly shocking.

"The Government urgently needs to investigate these findings and establish what has caused this sharp increase.

"We have long been calling for a national awareness and education campaign on bowel cancer and these figures further highlight the need for such a campaign."