A bombshell internal report written for the British meat industry reveals nitrites do not protect against botulism – the chief reason ham and bacon manufacturers say they use the chemicals.

The study, conducted for the British Meat Processors Association (BMPA) by the scientific consultancy Campden, and marked “confidential”, examines the growth of the toxin Clostridium botulinum in the processing of bacon and ham.

It is understood that the paper, seen by the Observer, was commissioned to provide evidence that nitrites, which have long been linked to cancer, are essential to protect consumers from food poisoning and, in particular, botulism, a potentially fatal disease. But, in what appears to be a major blow to the industry’s claims, the research found there was no significant growth of the bacteria in either the nitrite-free or the nitrite-cured samples that were tested.

The paper concludes: “The results show that there is no change in levels of inoculated C botulinum over the curing process, which implies that the action of nitrite during curing is not toxic to C botulinum spores at levels of 150ppm [parts per million] ingoing nitrite and below.”

It is illegal to add nitrites at levels above 150ppm due to health concerns: research has linked nitrites when added to meat, which is then cooked and ingested, to the development of cancer-triggering chemicals in the stomach.

This evidence raises serious questions about why nitrites are being added to our bacon and ham Baroness Walmsley

“This leaked internal report is highly embarrassing for the processed meat industry and for the Food Standards Agency which have persistently peddled the myth that nitrites are essential to protect against botulism,” said Baroness Walmsley, the vice-chair of parliament’s all-party group on cancer. On the contrary, this report reveals nitrites are not a controlling factor against Clostridium botulinum. This evidence raises serious questions about why nitrites are being added to our bacon and ham.”

Labour’s deputy leader Tom Watson said: “This damning report demonstrates there is no longer any need for the processed meat industry to be adding cancer-causing nitrites. We need to see the removal of nitrites from our food as soon as possible.”

Producers also use nitrites as it keeps bacon and ham pink for longer, thus more attractive to consumers. However, in a sign that consumers are becoming more aware about the threat posed by nitrites several companies, including Nestlé and Finnebrogue, now sell ham and bacon produced without nitrites.

Experts said the BMPA’s confidential research added to a growing body of evidence making the case against using nitrites.

Dr Aseem Malhotra, an NHS cardiologist and member of a coalition of experts calling for their removal, said: “The findings support guidance from the UK’s Advisory Committee on the Microbiological Safety of Food that has long excluded nitrites as one of the controlling factors required to prevent Clostridium botulinum growth and extend shelf life.”

Professor Denis Corpet, another member of the coalition and co-author of a 2015 World Health Organisation-sponsored study which first linked nitrite-cured processed meats to cancer, said that the addition of the chemicals to foods “marks out those meats that contain these chemicals as significantly more dangerous than other processed meats, such as traditional UK sausages or Italian Parma ham, which do not.”

In a letter last month to health secretary Matthew Hancock, Corpet wrote: “Since the study was published and endorsed by the World Health Organisation four years ago, there has been an unsatisfactory response from those who hold the levers of power – and that includes your administrations in the EU and UK.

“In the years since, more evidence has exposed other wide-ranging health risks posed by nitrites. Yet, despite these facts, those responsible for ensuring the food consumers eat is as safe as it possibly can be have failed to act.”The BMPA said it took safety very seriously. “The work referenced is still being reviewed and is subject to confidentiality clauses. It also needs to be further peer reviewed. The work should be seen as a demonstration of the industry’s total commitment to providing safe affordable and quality food to the consumer and leaked extracts without context are unhelpful. After proper scrutiny by peers this work will be presented to the Food Standards Agency to help shape future regulation.’’