UK farmers could be allowed to use powerful antibiotics in ways soon to be banned by the European Union, after the government was accused of using Brexit to avoid implementing tougher rules on animal health.

New rules aimed at curbing overuse of the drugs are being brought in by the European commission, but they will not come into effect before the Brexit cut-off date in March. The Guardian understands that government animal health experts have been advising vets and farmers they will therefore not have to implement the change.

A divergence from EU rules could allow farmers and vets in the UK to dose healthy animals through their feed, as well as those diagnosed with illnesses – while EU farmers will be prevented from doing so. Campaigners say this is irresponsible misuse of antibiotics that can lead to resistance and should be stopped in line with European rules.

At an event this summer held by the UK’s Veterinary Medicines Directorate (VMD), which regulates farm antibiotics, the VMD’s director of operations, Paul Green, said the UK would implement the new EU regulations “as fully as we see fit … there may be some clauses we wish to omit [or] alter”. According to a person present at the event, Green then went on to say the government would allow farmers in some circumstances to continue to mix antibiotics in feed and drinking water to groups of animals for disease prevention.

Antibiotic use in farming is controversial; livestock account for the majority of antibiotics consumed, and overuse on herds and flocks can build up resistant bacteria that can spread to humans. Rules on the medicines have been toughened by the EU over the past decade. Under the EU’s planned improvements, it would no longer be possible for vets and farmers to mix antibiotics with feed to be given to large numbers of animals at a time, except under exceptional circumstances. This is seen as essential to cutting the overuse of antibiotics, as currently whole herds or flocks can be treated at once because of one sick animal.

The government is coming under pressure from potential trade partners not to adhere to stringent EU animal welfare standards after Britain leaves the EU, in order to open up the UK market to imports from countries with weaker regulations.

The US Department of Agriculture, which has a strong influence on trade deals, has been working on plans that would allow groups of healthy animals to be dosed with antibiotics, against World Health Organization guidelines. These plans could play a key part in future trade with the US.

The Department for Environment, Food and Rural Affairs said: “Leaving the EU will not change our commitment to tackle antibiotic resistance and the UK will remain a world leader in helping preserve these medicines for future generations.”

Defra is holding another public meeting this week on the subject of agriculture post-Brexit, at which government representatives are likely to come under pressure to spell out the details of what Brexit will mean for farming.We are therefore not constrained by this legislation and with negotiations still ongoing, but we could even go further than this EU legislation if that is what we see fit.”

“The UK is a world leader in tackling antimicrobial resistance and our approach will not change when we have left the EU. We will use Brexit as an opportunity to enhance animal welfare wherever possible.”

Government claims that Brexit would not result in a weakening of food and farming standards were “not credible”, according to Keith Taylor, the Green party MEP. “Plans not to enforce vital EU rules on antibiotic misuse are extraordinary,” he told the Guardian. “It has been clear from day one that Liam Fox and the shadowy Brexit syndicate have set their sights on scrapping the EU safeguards currently protecting our environment, animal welfare and public health in order to secure a toxic TTIP-like trade deal with Trump’s US.”

He pointed to recent revelations in the Guardian over the use of drugs critically important for human health being given to animals in large quantities in the US, as an example of the dangers of straying from EU standards to pursue a US trade deal. “Pumping farmed animals full of antibiotics is bad for their welfare and an existential threat to human health,” he said. “Overmedicating livestock is a means by which the hugely harmful factory farming industry shortcuts the implementation of even the most basic animal welfare and biosecurity measures.”

Cóilín Nunan, of the Alliance to Save Our Antibiotics, told the Guardian: “This European ban on preventative mass medication is long overdue and has taken member states years to agree. It will be hugely important for protecting human health, so it’s shameful that the VMD is saying that the UK is going to avoid implementing it.”

He said the government’s claims to be a world leader on tackling antibiotic misuse were at odds with allowing “kind of irresponsible use” of the drugs in countries who could sign a post-Brexit trade deal with the UK. “What kind of trade deals will the UK be signing with countries like the US and China, which massively overuse farm antibiotics, if at the same time it has some of the lowest regulatory standards in Europe?” Nunan asked.

Overuse of antibiotics in farming is a major new threat to human health, says UN Read more

Antimicrobial resistance, by which organisms develop mutations that render antibiotic drugs ineffective, has been called a looming “apocalypse” by England’s chief medical officer, Dame Sally Davies. She has repeatedly warned that even routine operations will become dangerous or even impossible within as little as a decade, if the growth of resistance is not checked.

The World Health Organization has called for at least the most powerful antibiotics, the so-called drugs of last resort used to treat human diseases when nothing else is effective, to be banned for use in livestock. So far, however, countries have failed to follow this advice.

The Guardian has tracked the emergence of livestock-associated MRSA in the UK in the past few years, which scientists say is linked to the overuse of antibiotics. Though various farming organisations have embarked on programmes to reduce the use of antibiotics on farms, campaigners say there is still a long way to go, and that the government’s methods of monitoring the medicines’ use are outdated.

• This article was updated on 30 September 2018 to expand Defra’s comment in response to the article. Defra also say Paul Green denies making the comments described in the article.