The leaders of the UK’s main medical associations have taken the unprecedented step of writing to the government to call for changes in the way farm animals are medicated, out of fear that current practices are causing resistance to antibiotics used in human medicine.

Eleven of the UK’s most senior medics, including the presidents of the Royal College of Physicians, the Royal College of Surgeons and the Faculty of Public Health, have signed the letter to the secretaries of state for health and environment.

They are calling on the government to commit to a complete ban on the preventive use of antibiotics in animal treatment. The European parliament has voted for such a measure, but it will not come into force until 2022, after Brexit. This means the UK government will not be bound by it.

The doctors want an explicit commitment, saying government officials have been equivocal over whether the UK will bring in a full ban.

They warn that failure to do so would leave the UK with “some of the weakest regulatory standards for farm antibiotics used in Europe. This could seriously undermine the very welcome progress currently being made in reducing UK farm antibiotic use and negatively impact human health”.

Even when only one animal is sick, the whole herd can be treated as a preventive measure. But this risks the overuse of antibiotics on healthy animals, which is a prime cause of the increase in antibiotic resistance.

Prof John Middleton, president of the Faculty of Public Health, said: “A future world where bugs are all resistant to antibiotics will return us to the dark days of ineffective healthcare and condemn many to early deaths. Animal health and human health must be equally protected to save our antibiotics – that is why we’re making this call on government.”

He added: “In the post-Brexit world, it will be even more vital that we increase our standards on antibiotics use by doctors and farmers, so that the UK is a world leader, saving our antibiotics to save lives in future.”

Dame Sally Davies, the government’s chief medical officer for England, has repeatedly warned that the world faces an antibiotic “apocalypse”, in which common illnesses become untreatable and common operations become life-threatening.

Most of her warnings have been directed at the overuse of antibiotics in human medicine, but experts say the use of the medicines on farms is also a significant and growing danger. The same medicines can be used to treat livestock as humans, and resistance can be passed on.

For instance, the Guardian has revealed instances of livestock-associated MRSA appearing among the human population, and it is growing more prevalent on UK farms. A Guardian investigation found livestock-associated MRSA in supermarket meat samples, from where it could infect people.

Experts believe this is just the tip of the iceberg, and that farms are becoming a greater reservoir for antibiotic resistant bacteria, with little oversight and monitoring.

Coilin Nunan, of the Alliance to Save Our Antibiotics, which coordinated the letter, warned that any future free trade deals with the US could also pose a risk. He said: “If the government fails to implement a ban on group prevention, the UK will be aligning itself with the US administration’s position, which is to strongly oppose the European ban. This should raise alarm bells about the kind of post-Brexit trade deal the UK may agree with the US, where antibiotics are used in enormous quantities in livestock.”

A Defra spokesperson said: “We recognise the importance of tackling antimicrobial resistance and are fully committed to continue to lead the global fight. Measures in the upcoming agriculture bill will support the delivery of this work and help to reduce the need to use antibiotics in a way that safeguards the health and welfare of our farmed animals.”

Antibiotic use in US farming is six times higher per livestock unit than in the UK, according to the alliance, and cattle in particular receive 13 times as much antibiotic medicine as in the UK.

Earlier in November, the former environment secretary Lord Deben warned that UK food standards would be put at risk in any likely free trade deal with the US.

Farming groups and vets in the UK have succeeded in reducing antibiotic use on UK farms in recent years, but this progress could be at risk, the alliance warned. More antibiotics are used worldwide in farming than in treating people. In the UK, livestock account for 40% of all use.

The organisations signing the letter are: the Royal College of General Practitioners; the Royal College of Physicians; the Royal College of Surgeons; the Royal Society of Medicine; the British Medical Journal; the Lancet; the Faculty of Public Health; the Royal Pharmaceutical Society; the Royal College of Physicians and Surgeons of Glasgow; the Royal College of Paediatrics and Child Health; Medact; and the Alliance to Save Our Antibiotics.