The research will be welcome news for Britons who spend an average £62 a year each on chocolate.

Doses used in the experiments were similar to the amounts that someone might eat if they snacked on chocolate occasionally.

The discovery is just the latest benefit of antioxidants to be uncovered – including cutting the risk of heart disease.

Last month German scientists gave hope to diabetes victims, finding that a special form of enriched cocoa might reverse some of the damage done to sufferers’ arteries.

The latest study – by experts at Georgetown University in Washington DC – makes the case for some of the properties of chocolate as a cancer treatment. Bowel cancer is the second biggest cancer killer, after lung cancer, in Britain, with one in 18 being diagnosed with the disease. 

Half of those diagnosed – or 50 people a day – will die.

Researchers tested GECGC, a synthetic version of the chemical made by confectionery giant Mars, which partly funded the study. They exposed 16 different types of cancer cells to the chemical and found that it had an effect on four.

The results, published in the journal Cell Cycle, showed that the strongest response was seen in two types of bowel cancer cells, where tumour growth was slowed by 50 per cent – and malignant cells damaged by the chocolate chemical. But it remains unclear exactly how the chocolate chemical works on tumours. 

One theory is that it blocks the signals to and from the cells that are needed to help them to grow. But researchers stressed that the chemical had no impact on ovarian or prostate cancer cells.

Dr Min Kim, who led the study, said: “We have all heard that chocolate is good for you. Now this study suggests one reason why that might be true.

“This chemical seems to be safe, which makes sense because it has a structure similar to a natural product in cocoa beans.”

A spokeswoman for the charity Beating Bowel Cancer said: “The Georgetown research is encouraging.

“We urge people to maintain a healthy, balanced diet, containing plenty of fibre, combined with regular exercise to help to prevent bowel cancer.”

Procyanidins are also found in apples and wine.

The French National Institute for Health and Medical Research has found that apples could aid the fight against colon cancer because of high concentrations of procyanidins in apple skin.

Experts at London’s Queen Mary University compared the amount of procyanidins in red wine to pinpoint which ones to drink to help you to live longer. 

Red wines from South-west France or Sardinia were found to be the healthiest.