Hormone-fed beef and chlorine-washed chicken should remain banned in England after Brexit, the government has been warned.
Ministers say the issue will be dealt with in the upcoming Trade Bill.
But opponents of these practices say that could lead to farm standards being bargained away in negotiations.
Instead, they want ministers to guarantee food standards in the Agriculture Bill, which returns to the House of Commons on Wednesday.
Some Conservative MPs have joined up with the opposition to demand protection for England’s farmers from lower standard produce from countries like the US.
Farmers there are allowed to feed beef with hormones and wash chickens with chlorine solution in order to maximise productivity.
But both of these practices are currently banned in the EU. The US demands that ban should be lifted.
The issue is part of a great upheaval in UK farm and countryside policy – the biggest since World War II.
The UK government wants to shift farm grants to reward activities that enhance the environment.
Its opponents are concerned at the lack of clarity over exactly how the transformation will happen, and want the changes to be delayed.
Minsters are likely to suffer huge pressure in the Commons on the question of food import standards.
They face almost identical amendments to the Bill from some Conservative back-benchers; the Environment, Food and Rural Affairs (Efra) Select Committee; and the Labour and Liberal Democrat front benches. They are all calling for a level playing field on food standards.
Jyoti Fernandes, from the Landworkers’ Alliance, a union of farmers and other land-based workers, said: “The Agriculture Bill is a historic moment to make or break our food system.
“If we don’t protect our farms from being undercut by cheap imports and make a firm commitment to supporting our farmers through this transition, we can wave goodbye to a humane and ecological domestic food supply for future generations.”
Sally-Ann Spence, from the Nature Friendly Farming Network, runs a farm in Wiltshire. She said: “As a farmer doing my best to protect our precious natural environment and heritage, the prospect of low-standard imports fills me with dread.
“In the UK, we are striving to deliver healthy food at world-leading standards whilst managing the land for wildlife and public goods.
“I urge MPs to safeguard our high environmental and animal welfare standards in trade law.”
With its huge majority, the government is not thought to be in danger of losing key votes.
But Neil Parish, Conservative chair of the Efra committee, said: “We (Tories) put high welfare standards in our manifestoso people will be expecting us to deliver on that.
“The government mustn’t allow any trade deal with the US, or anyone else, to undermine British food standards.”
The BBC has contacted the government for a comment.
Agricultural policies in the UK are devolved.
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