The variety has higher levels of sulphoraphane and will help the half of the population which lacks a gene that allows the body to retain the protective plant chemical, they said.

Prof Richard Mithen, of the Institute of Food Research in Norwich and the lead scientist on the project, said yesterday: "Eating a few portions of broccoli each week may help to reduce the risk of cancer."

He said half of people appear to get less cancer protection from broccoli because they lack the gene GSTM1.

"Our studies suggest that this may be because if you lack the gene you cannot retain any sulphoraphane inside your body, it is all excreted within a few hours.

"But if you consume larger portions of broccoli, or broccoli with higher levels of sulphoraphane, such as the super broccoli, you may be able to retain as much sulphoraphane in your body as those who have the gene. Broccoli is also a rich source of other vitamins and minerals."

The super broccoli, which has been developed by traditional plant breeding methods, has 3.4 times more sulphoraphane than standard varieties. It has the same taste as regular broccoli and could be on sale "in a couple of years," Prof Mithen said.

Broccoli belongs to the crucifer family of plants which includes the brassicas cabbage, cauliflower and sprouts, as well as the closely related Chinese cabbage and turnips.

Crucifers contain high levels of glucosinolates, which release isothiocyanates - some of the most potent dietary anti-carcinogens known. Sulphoraphane is the main isothiocyanate derived from broccoli.

Although people who lack the GSTM1 gene may get less cancer protection from eating broccoli, it is likely that they will gain more protection from eating other types of crucifers. "So the best advice is to eat a mixture of crucifers," said Prof Mithen.