A roar of laughter erupts from a crowd inside a large conference room at the De Vere Cranage Estate hotel. The joke? The idea that visiting the doctor could help treat a serious illness.

On stage the controversial Australian proponent of natural remedies Barbara O’Neill is taking part in a question-and-answer session during a five-day wellness conference, where science mixes uncomfortably with Bible verse and potentially deadly pseudo-scientific practices.

She is advising people attending at the event in Cheshire on a variety of medical ailments, and dismisses the use of antibiotics, vaccines or scientifically proven cancer treatment.

Tickets for the residential retreat can cost up to £1,300. An undercover reporter for the Guardian attended for one day, observing four sessions covering topics including the immune system and one titled Barbara Under Attack.

The audience’s rapturous laughter is prompted by a woman asking O’Neill if there is anything that could help her lupus, which she says has not responded well to the doctor’s treatments. “Yes, keep away from the doctor,” quips O’Neill.

O’Neill tells the audience that she is “just a little lady”, but she is an incredibly influential one, with 3.6 million Instagram followers and 1.7 million on Facebook. Many of her videos rack up millions of views.

Barbara O'Neill. Photograph: Facebook

Having initially found success promoting alternative treatments in Australia, in 2019 O’Neill was banned from providing any health services in her home country, where authorities found she was responsible for “misleading vulnerable people”. They said she “made dangerous claims not supported by evidence and posed a risk to public safety”.

She has since taken her unregulated services overseas, most recently with a string of appearances in the UK and Ireland which prompted concern from experts in both countries. A number of prominent doctors and scientists have told the Guardian her advice is “dangerous”, with some warning it could kill.

O’Neill appears regal sitting on stage in an armchair offering audience members softly spoken advice on endometriosis, varicose veins and constipation.

The 73-year-old is dressed in a long skirt with a woollen wrap, which she later switches for a soft cardigan, creating a look that sits somewhere between Queen Elizabeth II at Balmoral and Claudia Winkleman in The Traitors. The Highlands-inspired attire is a nod to her Scottish heritage, which the Australian mentions on multiple occasions.

She carries an air of authority to her devoted followers, many of whom seem confident that her doctrine could improve their health.

Much of her core teaching aligns with established scientific health advice: few doctors would dispute that a balanced diet, regular exercise, hydration and plenty of sleep are important factors for good health, while it is advisable to avoid alcohol, caffeine and smoking.

During some of the lectures, O’Neill draws diagrams on a whiteboard while issuing long scientific-sounding explanations.

She talks about the glycocalyx lining of the endothelium cells, which provides anticoagulant effects and releases nitric oxide, a vasodilator.

But she also makes some highly unscientific and potentially deadly recommendations.

‘It’s amazing how many tests are creating illnesses,’ O’Neill said at the Cheshire event. Photograph: Robyn Vinter/The Guardian

During the Q&A session, a woman says that while she recently underwent tests, doctors found a stomach tumour, which she says two specialists have advised her to have surgically removed.

“Sometimes scans are not right,” replies O’Neill, who advises the woman to treat the tumour with a castor oil compress – kitchen roll soaked with high viscosity oil from the castor plant – on the outside of the body.

That day O’Neill recommends the same remedy for nasal polyps, endometriosis, carpal tunnel syndrome and bile duct stones. Medical experts told the Guardian there was no evidence castor oil compresses did anything to improve any of those conditions.

She suggests using castor oil with a pinch of cayenne pepper in the eye to an audience member with glaucoma and high blood pressure.

Prof Gus Gazzard, a leading glaucoma expert, says: “There is absolutely no evidence to support these claims, and unfortunately, there’s the possibility that they might indeed cause harm.”

The questions continue, read out by her husband, Michael, who gives some of his own lectures during the retreat. The pair stumble over long names of medical conditions, while the audience laughs along.

O’Neill advises a pregnant woman against taking folic acid, which randomised controlled trials have shown can protect against neural tube deficits in babies. To someone else, she claims antibiotics are “the worst thing to give” for infections.

Jewellery at O’Neill’s conference in Cheshire. Photograph: Robyn Vinter/The Guardian

To a woman who says she does not have a functioning thyroid, she recommends slowly stopping her dose of thyroxine, which is prescribed for thyroid dysfunction, to “see what the body does”.

Medical experts say this would lead to fatigue, hair loss, weight gain, a slow heart rate and low mood. Martin Whyte, a professor of metabolic medicine at the University of Surrey, says: “Someone without a functioning thyroid cannot make thyroid hormone, so stopping thyroxine to ‘see what the body does’ really makes no sense and risks serious, potentially life-threatening illness.”

A woman who says she has diabetes tells O’Neill she is hoping to come off metformin, a drug used to lower blood sugar levels. Metformin is typically used to treat type 2 diabetes, which can respond to a change in diet, while type 1 requires daily insulin injections. Both types require blood glucose monitoring. O’Neill advises her to stop testing her blood sugar because it would “balance right out with the change in diet”.

This advice is “very dangerous”, according to Gill Livingston, a professor of psychiatry of older people at UCL. “Untreated diabetes means the body is bathed in bloodstream high sugar and increases the risk of multiple physical illnesses, such as loss of eyesight, loss of peripheral sensation, dementia and sometimes death.”

Refusing medical tests is a regular theme in O’Neill’s teachings across her publications, her social media and her public appearances. “It’s amazing how many tests are creating illnesses,” she says at the Cheshire event.

At another point, not long after encouraging a follower to ignore her oncologist’s recommendation to take a drug, and without any clarification about the type of cancer the woman was being treated for, she says: “They give you a label and then they give you a drug.”

Interspersed throughout the day are claims that people have recovered from illnesses after taking her advice. She cites a man who had suffered a stroke, and had “no adverse effects at all” when he stopped taking medication prescribed by his doctor, including statins and blood thinners, on one of her retreats.

“It’s outrageous what medicine will do. We have to start saying ‘no, thank you’,” she says. “It doesn’t matter what they say to you, what fear they put into you, say no.

“The body, it’s got an inbuilt ability to heal itself.”

Prof Liam Smeeth, director of the London School of Hygiene & Tropical Medicine, calls her claims “extraordinary”. He says: “Most disturbingly, many of these – if followed – will cause substantial harm, and this could include death.”

A product that claims to protect against EMF. Photograph: Robyn Vinter/The Guardian

It is clear some of her followers have a lot of trust in her. Speaking to the Guardian’s undercover reporter at the end of the day, one audience member says O’Neill’s answer to her question was “very helpful” and she now has the confidence to go against specialists’ advice for surgery for a health condition.

In other conversations during breaks, some people attending say they have lost trust in western medicine after side-effects or failed treatments.

One woman in her 60s explains that she and her family refused masks and vaccines during the Covid crisis, had gatherings and “actually washed my hands less than normal”. She says her husband went into hospital in early 2020 with pneumonia and died two days later. She blames the medics for his death.

“You know they’re satanists, don’t you?” she says, of the government and those who run major global institutions. “The elite are all flesh-eating paedophiles.”

Some at the conference wear amulets they claim protect against 5G signals and a stall selling them proves popular. There is no evidence that 5G signals cause harm and yet O’Neill advises those working at a computer to invest in a device to protect against damage she says is caused by electronic devices and “balance the body after exposure to an electromagnetic field [EMF]”.

Dr David Robert Grimes, an expert in biostatistics at Trinity College Dublin, says fear of modernity is a common pseudoscience trope.

“The canard that EMFs are ‘damaging’ us is a tired one that exploits our ignorance of technical terms to instil fear,” he says. “There is no device that ‘balances’ you after exposure, because the central conceit is nonsense. We live in a world with light, heat, radio waves, and the sun, so EMFs are all around us. The idea of balancing is really meaningless.”

He adds: “The evidence is clear that patients seduced by the false allure of complementary medicine fare objectively worse than those that stick to conventional treatments. These patients are typically convinced by dubious healers to delay or reject life-saving treatments until it’s too late, with often tragic consequences.”

Before O’Neill’s most recent tour, senior doctors and medical experts, including Grimes, wrote to some of the venues hosting her conferences urging them to reconsider holding events that would “spread dangerous health fictions to a susceptible audience”.

O’Neill’s husband, Michael, co-runs her company and is the engine behind her tours. In an afternoon session titled Barbara Under Attack, he describes her “persecution” by authorities, the scientific community and the media . “There is a war against us,” he tells the audience.

After an approach from the Guardian to O’Neill for comment on her claims at the conference, a video of the couple appears on her Instagram and Facebook pages warning their followers of “disgusting” reporting. Michael reads out the statement issued to the Guardian.

“The complaints against Barbara are purely speculative, based on an ideology that the only legitimate way to health is through the medical profession,” he says.

Michael claims the couple have not received complaints about O’Neill’s work and highlights statistics about medical mistakes in the NHS.

He adds: “The campaign against Barbara by the media, and academics is a modern-day witch-hunt that has no substance in facts, but purely an ideology that supports the most corrupt organisation in the world; the pharmaceutical industry.”

The pair have vowed to continue holding events. O’Neill says she is driven by God, who she believes spoke directly to her.

“It would be wrong of me to stop.”