The benefits of supplements containing antioxidants such as vitamins A, C and E are the subject of fierce debate.

While some studies suggest they could offer moderate protection against cancer, Professor Watson, who with Francis Crick discovered the ''double helix'' structure of DNA in 1953, argues the pills could do more harm than good. In a new paper, he claims that the reason late-stage cancers often become untreatable is that they produce high levels of antioxidants which stop treatments such as chemotherapy and radiotherapy from working.

In healthy people, antioxidants can be helpful because they attack molecules known as ''free radicals'' which can damage DNA. But many cancer treatments use free radicals to kill tumour cells, meaning antioxidants could prevent them doing their job.

Professor Watson said studies should be carried out to test his theory, which he described as ''among my most important work since the double helix''.

Writing in the Royal Society's Open Biology journal, he said: ''For as long as I have been focused on the understanding and curing [of] cancer, well-intentioned individuals have been consuming antioxidative nutritional supplements as cancer preventatives if not actual therapies.

''In light of the recent data strongly hinting that much of late-stage cancer's untreatability may arise from its possession of too many antioxidants, the time has come to seriously ask whether antioxidant use much more likely causes, than prevents, cancer.''

Professor Nic Jones, Cancer Research UK's chief scientist, said: ''We know from many large studies that, far from being potent cancer-fighters, antioxidant supplements seem to be ineffective for cancer prevention in healthy people, and some can even slightly increase the risk of cancer.''

Cancer Council Australia chief executive Ian Olver said the comments ''had merit''.

''Multivitamins are something that may be helpful in prevention, but they may not be helpful for treatment,'' Professor Olver said.

Patients and doctors should ensure alternative medicine use is openly discussed, he said.

Dr Wendy Morrow, executive director of the Complementary Healthcare Council of Australia, agreed that doctors should ask patients about their use of complementary medicine, including multivitamins.

She said: ''What Professor Watson is doing is providing a theory. But to make a statement that multivitamins can't be used with a treatment is a bit of a stretch.''

The sharemarket listed Blackmores and Swisse as Australia's two largest vitamin and supplement sellers, a market analysts say is growing by 5 to 6 per cent a year.

A Swisse spokesman declined to comment on Professor Watson's report, saying it was against the Therapeutic Goods Administration's regulations for Swisse to talk about cancer treatment.

TELEGRAPH, with MADELEINE HEFFERNAN, JULIA MEDEW