Campaigners have warned that the next government must clamp down on the overuse of antibiotics in farming to stave off increasing risks of resistance that are threatening human health.

Brexit threatens to open up loopholes in the UK’s regulations that would mean diverging from strengthening EU standards on antibiotic use in farming, and the pressure from potential trade deal partners will be to lower standards.

Cóilín Nunan, scientific adviser at the Alliance to Save Our Antibiotics, told the Guardian: “Any future government must implement a complete ban on preventive mass medication [of livestock] as a step towards sustainable and responsible farm antibiotic use. Future trade deals should only allow imports produced to UK antibiotic and animal-welfare standards and import tariffs should reflect the benefits of higher-welfare systems, such as grass-fed or organic.”

He warned: “The Boris Johnson deal with the EU makes weaker commitments on maintaining regulatory parity with the EU than the previous May deal did. The government has made it clear it wants to be able to set its own regulatory standards.”

Antibiotic use on farms in the UK is controlled in line with EU regulations, but those could be loosened in the event of Brexit. There have already been signs of divergence, according to the Alliance to Save Our Antibiotics, as the government has to date not committed to a strengthening of the rules that will be enforced EU-wide in 2022.

Nunan added: “The government needs to do far more to encourage a shift towards better animal husbandry to improve animal health and welfare, so that there are fewer infections in the first place. Sweden, which has higher minimum animal-welfare standards, has the lowest level of farm antibiotic use in the European Union.”

Problems include the very early weaning of piglets, and the very high stocking densities of poultry, which both lead to higher antibiotic use that could be lessened with higher welfare measures.

The call comes on the 50th anniversary of a landmark report on antibiotics, the Swann report, that found serious dangers with the overuse of the drugs. The rules are in danger of being watered down by Brexit.

Antibiotics of the strongest kind, the last resort for human health, are still allowed to be used in farming, and the data kept on antibiotic use in the UK does not permit a detailed breakdown of how the drugs are used. Powerful antibiotic drugs are also available for unlicensed import over the internet, though they are only supposed to be used under veterinary supervision.

Agriculture would form a key plank of any new trade deal, but major exporters such as the US and many south American countries have less strict controls on the use of the drugs than apply in Europe.

The Swann report found that the intensification of livestock farming had led to more disease problems and failed to find “any excuse in logic or theory” for the preventive dosing of herds or flocks with the drugs.

Nunan said: “Fifty years ago, the Swann committee gave in to pressure from vested interests and failed to recommend an end to preventive mass medication, even though it knew the practice was unjustifiable and the cause of a deadly outbreak of antibiotic-resistant salmonella. Half a century later, still no one has come forward with a valid reason for allowing this misuse of antibiotics to continue.”

Routine use of antibiotics became popular with farmers, because it can lead to animals putting on weight faster. But such overuse spurs the evolution of resistant germs, leading to a ban on routine antibiotic use for growth-promotion in the UK, the EU and some other countries in the decades since the report.

Use of antibiotics in the UK has halved in recent years, but is still higher than before the publication of the Swann report. Some trends are also causing concern among experts. For instance, in the poultry industry, large reductions in antibiotic use have been made, but the use of a class called ionophores has soared to record levels in the last five years. These are not currently used in human medicine, but could be in future. The use of zinc oxide is also to banned at high doses in piglet feed by the EU in 2022, but the UK may not follow the ban.

More antibiotics are used on livestock than on people, even though resistant germs can be passed back to humans from animals. The outgoing chief medical officer for England, Dame Sally Davies, has warned repeatedly that growing resistance to antibiotics is now one of the biggest threats to human health, and that it could render routine operations life-endangering.

Molly Scott Cato, the Green party MEP who helped push strengthened regulation through the European parliament, said: “Unhealthy, intensive livestock farming relies on routine antibiotic use but this also poses a risk to human health by encouraging the development of resistant bacteria. The US still has much looser regulation and we can expect any potential trade negotiations if Brexit goes ahead to include pressure on antibiotic restrictions as with other farm safety standards. Our weak bargaining position will make it unlikely that we can resist the pressure, leading to risks to both public heath and animal welfare.”

The government’s record was defended by Lord Gardiner, the Tory spokesman on farming, who said: “Strong progress has been made in reducing the use of antibiotics in the livestock sector. We are doing more than many EU countries and Brexit will not change that.”

Jane Dodds, the Liberal Democrats’ spokeswoman on farming and rural affairs, said: “As part of the European Union, the UK has made significant reductions in its use of antibiotics in livestock farming – with significant reductions since 2012. The Tories are now risking squandering that progress. Their obsession with a trade deal with Donald Trump would see a bonfire of environmental regulations and animal welfare standards, undermining British farmers.”

Labour’s Sue Hayman, the shadow minister for environment and rural affairs, said: “Time and again the Tories have voted down attempts by Labour to enshrine trade deal assurances on animal welfare, health and environmental standards in law. It is clear we can’t trust the Tories to uphold our current standards on antibiotic use in farming.”