Feeling she had nothing to lose, Prof Plant switched to a dairy-free, Asian-style diet virtually overnight, while also undergoing chemotherapy. Having already cut down on animal protein such as meat, fish and eggs, she now cut out all milk products, including the live organic yogurt she had religiously eaten for several years.

Within six weeks the lump in her neck had disappeared; within a year, she was in remission and remained cancer-free for the next 18 years. Convinced that her diet had helped, she devised the Plant programme – a dairy-free diet, relying largely on plant proteins such as soy – similar, she says, to the traditional diet in rural China.

It was originally intended to help other women with breast cancer and, later, men with prostate cancer. Her book about her experience, Your Life in Your Hands, caused a sensation when it was published in 2000, with many cancer patients claiming it helped them to recover.

But in 2011, Prof Plant’s breast cancer returned for the sixth time, with the discovery of a large lump beneath the collarbone and some small tumours in her lungs. Under stress writing an academic book, she had become lax about both her diet and lifestyle – regularly eating, among other forbidden items, calves’ liver cooked in butter at a restaurant, and falafel made from milk powder.

“I went straight back to my oncologist, who prescribed letrozole [an oestrogen suppressor]. But I also went back on my strict diet, as well as walking regularly and doing meditation.” After a few months, her cancer was again in remission.

All of which may sound too good to be true, but Plant, 69, is no crackpot. Professor of geochemistry at Imperial College London, where she specialises in environmental carcinogens, she is highly regarded in her field, having been awarded a CBE in 1997 for her services to earth science; and her approach to cancer is supported by some eminent scientists. Her latest book, co-written with Mustafa Djamgoz, professor of cancer biology at Imperial, has a foreword from Prof Sir Graeme Catto, president of the College of Medicine, who describes its findings as “illuminating… even, at times, shocking” but all backed up by scientific research.

Prof Plant, however, is not dismissive of conventional cancer treatment, having had, at various times, a mastectomy, chemotherapy, radiotherapy and irradiation of her ovaries to induce menopause.

She believes new and “wonderful” anti-cancer treatments are vital – but so, she argues, is a dairy-free diet, as well as other diet and lifestyle measures, such as stress reduction.

Much of the advice in the new book, Beat Cancer, chimes with current guidance on how to reduce cancer risk, such as eating more plant food and less red meat, salt, sugar and fat; taking regular exercise and reducing stress.

She also advises going organic, using complementary therapies where there is good evidence they help recovery, and avoiding potential pollutants such as pesticides.

But her far more radical message is that a diet that totally excludes dairy products – milk, cheese, butter and yoghurt – can be successfully used to help stop the disease “in its tracks”, by depriving cancer cells of the conditions they need to grow.

“We have all been brought up with the idea that milk is good for you,” says Prof Plant. “But there is evidence now that the growth factors and hormones it contains are not just risky for breast cancer, but also other hormone-related cancers, of the prostate, testicles and ovary.”

Going dairy-free, she says, may also help patients with colorectal cancer, lymphoma and throat (but not lung) cancer. “Cows’ milk is good for calves – but not for us,” she adds.

With the relatively new science of epigenetics, scientists now understand that cancer-causing genes may not become active unless particular conditions arise that switch them on – and if those conditions change, they may be switched off. “This means that what you eat can have an impact at the genetic level,” says Prof Plant.

Cancer cells, scientists now believe, are hypersensitive to chemical messenger proteins called growth factors, as well as (in the case of hormone- dependent cancers) hormones such as oestrogen. Produced by our own bodies, growth factors perform vital tasks such as making cells grow. Other substances called binding proteins normally control them, including their potential impact on cancer cells. The risk of cancer arises when we have abnormally high levels of “unbound” growth factors (or hormones) circulating in our blood.

This can happen, say Profs Plant and Djamgoz, because the same growth factors and hormones as we produce are found in food that comes from animals, providing the very “fertiliser” that cancer cells need. Casein, the main protein in cows’ milk, is considered most dangerous. One eminent US nutritional scientist, Prof Colin Campbell at Cornell University, argues that it should be regarded just like oestrogen – as a leading carcinogen.

“Cow’s milk [organic or otherwise] has been shown to contain 35 different hormones and 11 growth factors,” says Prof Plant. High circulating levels of one such growth factor in milk, called IGF-1, is now strongly linked to the development of many cancers. Research has also found that “unbound” IGF levels are lower in vegans than in both meat-eaters and other vegetarians.

“This means that a vegan diet is lower in cancer-promoting molecules and higher in the binding proteins that reduce the action of these molecules,” she argues.

A second growth factor implicated in cancer spread is VEGF, found at high levels in cancer patients and a target for some newer anti-cancer drugs. Prof Plant points out that in the udders of cows with mastitis, VEGF is present to help fight infection. Mastitis is thought to affect nearly half of all cows in Britain. “There are increasing numbers of papers about high levels of VEGF in milk, particularly from high- yielding cattle breeds typical of modern industrialised dairy units.

“It seems likely that if a cancer patient is consuming dairy products, they are also consuming VEGF, especially if the milk originated from cows with mastitis. That is not helping to defeat their illness – and it may be making things worse.”

She is particularly worried about the fashion for high- protein diets, pointing out that there is evidence that too much protein generally – particularly from animals – is “at best unhelpful and at worst dangerous to those at risk of cancer”.

A second theory around diet concerns the levels of acid in our bodies. Prof Plant explains that if we consume too much acid-generating food, our bodies become acidic – an environment in which cancer cells can flourish. The foods highest in generating acid (not, as might be assumed, citrus fruit) include eggs, meat, fish and dairy – with cheese the most acid generating-food of all.

For those with cancer or at high risk of the disease, Prof Plant advocates, among other things, cutting out all dairy – from cows, sheep and goats, and whether organic or not. “If you have active cancer, there are no half-measures here.”

She also recommends limiting consumption of other animal protein, such as meat, fish and eggs, replacing this with vegetable protein such as soya – the main source of protein, she points out, in a traditional, rural Chinese diet.

But if the evidence that cutting out dairy can successfully “beat cancer” is that strong, why haven’t we been told?

Prof Plant puts it down to vested interests – the dairy industry represents about 12 per cent of Britain’s GDP – and medical conservatism: oncologists, she says, “might be excellent at conventional treatments but are not experts in nutritional biochemistry”. The big cancer charities, for their part, place too much emphasis on drug development. As a result, “if you rely solely on the cancer prevention advice from government, charities, health professionals or the media, you will be missing out on vital and potentially life- saving information.”

Cancer Research UK argues that so far studies investigating a link between cancer and dairy products have not given clear results.

“There’s no good evidence to support avoiding all dairy with the aim of reducing cancer risk,” said Martin Ledwick at Cancer Research UK. “It isn’t known if avoiding dairy plays a role in stopping cancer coming back. Patients should speak to their doctor or a qualified dietician before making any changes to their diet.”

Prof Plant acknowledges that advising cancer patients – and anyone keen on prevention – to change what they eat so radically is “a big ask”. Yet her own menu for that day – Weetabix and soya milk with molasses and linseeds for breakfast, wholegrain bread with hummus and salad for lunch and for that night, minestrone soup with cannellini beans, followed by pasta with homemade tomato sauce – is not so alien.

“People always worry about where they will get calcium if they give up dairy,” she says. “But you can get it from many plant sources.” Growth factors and hormones should be labelled on all dairy products, she argues, although eventually a wholesale shift away from dairy is needed.

Approaching her 70th birthday, Prof Plant has so far survived 27 years and six diagnoses of cancer and is a pretty convincing advert for the diet she advocates. Her story, though, has a sting in its tail: two weeks ago, a scan undertaken for a broken collarbone picked up two small secondaries, one in each lung. She is now taking tamoxifen and seems confident that a combination of medical treatment, diet and relaxation will knock this recurrence on the head.

“As a scientist, all I can do is tell the truth based on the evidence,” she says. “I started my first book because I didn’t want my daughter [Emma, now 39] to go through what I went through. All my books have come out of not wanting this to happen to others.”

‘Beat Cancer: The 10-Step Plan to Help you Overcome and Prevent Cancer’ by Prof Mustafa Djamgoz and Prof Jane Plant is published by Vermilion (£14.99)

THE ‘BEAT CANCER’ DIET

Beat Cancer advises anyone with cancer or at high risk of the disease to cut out all dairy products, organic or not, from cows, sheep, goats and all other animals. Replace:

Dairy milk with almond, coconut, rice or soya milk

Hard cheese with tofu or bean curd for sauces, soft cheese with hummus

Dairy yoghurt with soya or coconut yoghurt

Crème fraiche, fromage frais and cream with coconut or soya cream

Butter and margarines containing dairy with soya spreads, hummus, peanut or other nut or seed butter

Dairy ice cream with soya, coconut ice cream or other dairy-free types; milk chocolate with dark chocolate

Other advice includes replacing refined and processed oils with

extra-virgin olive oil; refined and man-made sugars with raw cane sugar; refined white bread, pasta and rice with unrefined wholegrain products; and cutting out preservatives and artificial flavourings and colourings.

Consumption of meat, fish and eggs should also be limited. Instead, eat unrefined carbohydrates, beans, nuts, vegetables and fruit. Salt is best replaced by herbs, and coffee by homemade juices, tap water and herbal tea.