Tomatoes that have been genetically modified to be rich in antioxidants can give protection against cancer, a team of British scientists has found.

Researchers at the John Innes Centre in Norwich created the crop of purple tomatoes by altering them with genes from snapdragon flowers. In tests, mice that were prone to cancer lived almost a third longer if their diet was supplemented by the modified tomatoes.

The findings, which appear in the journal Nature Biotechnology, pave the way for a new generation of "functional foods" that could potentially offer protection against serious diseases.

The team, led by Cathie Martin, a professor of plant biology, plans to investigate whether the anti-cancer properties of the tomatoes also protect humans.

The scientists started by plucking genes from snapdragons that are responsible for the deep red pigment in the flower's petals. The colouring is due to an antioxidant called anthocyanin, which is also found in blueberries and blackberries. By inserting the genes into tomato plants, the scientists created a crop so rich in anthocyanin it turned purple as it ripened.

Martin said she was "bowled over" by the results. "It's really clear that the mice eating the modified tomato start to die significantly later than mice on the normal tomatoes," she said. "At the very least, it suggests that what you eat can really make a contribution towards preventing disease and helping to keep you healthy."

Derek Burke, former chair of the UK's regulatory committee on GM, said: "This is a truly positive outcome from genetic modification of plants, and a real help to people wanting to improve their diets."