The findings, by researchers at the University of Illinois in America, support theories that chemicals in different foods interact to preserve health.

John Erdman, a professor of food science and nutrition who led the study, said: “Separately these two foods appear to have enormous cancer-fighting potential. Together they bring out the best in each other. We decided to look at these foods in combination because we believed it was a way to learn more about real diets eaten by real people.”

Both vegetables are known to have cancer-fighting qualities. Tomatoes have already been hailed as a defence against prostate cancer and broccoli is also thought to prevent tumours.

Scientists believe that lycopene, which turns tomatoes red, and glucosinolates, contained in broccoli, are responsible for the beneficial effects but have so far been unable to replicate their effects in supplement pills.

During the tests, scientists injected laboratory rats with human prostate tumours, then fed them diets of powdered tomatoes, dried broccoli, or a combination of the two. A fourth group of rats was given finasteride, a drug being tested for its potential to prevent prostate cancer.

The animals developed growths, but in those given food supplements they grew more slowly and stayed smaller than in those given the drug. Rats fed with a combination of tomatoes and broccoli had the smallest tumours of all.

This week the Department of Health will publish new targets with the aim of reducing cancer in people under 75 by 20%.

Times Recommends

• An officer and a gentle woman
• Dame Helen Mirren is worthy of the Epidaurus
• Foods that can help to prevent swine flu