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Supermarkets face growing pressure to guarantee they will ban hormone-treated beef and chlorinated chicken if a US-UK trade deal opens the market to American agriculture. 

Lib Dem MP Layla Moran has written to supermarkets including Tesco, Asda, Sainsbury’s, Iceland and Morrisons asking bosses to confirm that, if a deal is struck, they will not stock foods that do not meet current UK standards. 

It comes after Waitrose chief executive James Bailey said stores would not sell certain US foods, telling customers that “any regression from the standards we have pioneered for the last 30 years would be an unacceptable backwards step”.

Backbench MPs and consumer groups have been pressuring the government to guarantee in legislation there will be no lowering of food standards post-Brexit. 

In response, trade secretary Liz Truss has pledged to set up a new trade and agriculture commission to scrutinise future trade deals. 

She said the body would offer “advisory” recommendations to ensure British farmers “do not face unfair competition and that their high animal welfare and production standards are not undermined”. 

But there are fears the government could still overrule the commission. 

Moran, who is running to become leader of her party, told HuffPost UK consumers were “rightly concerned”, adding: “I am urging supermarkets to listen to these concerns, follow Waitrose’s courageous stance on this issue and confirm they will not sell products like chlorinated chicken or hormone-injected beef.

“In the absence of a cast-iron legal protections to ensure UK standards are not lowered, it’s important that supermarkets reassure people that these foods that won’t be stocked on their shelves.” 

So far, Waitrose is the only chain to back a ban. 

A survey by the consumer group Which?, however, found that 74% of people are opposed to importing food produced to lower standards and that 95% think it is vital to maintain existing standards. 

Sue Davies, head of consumer protection and food policy at the organisation, welcomed Waitrose’s stance, and said: “But it should not be left to supermarkets to keep food produced to lower standards out of our food chain – the government should put its commitments into law via the Agriculture or Trade Bill to reassure consumers that food standards will not be compromised in any trade deal.”

Meanwhile, the food and farming campaign group Sustain has also written to supermarkets calling on them to offer guarantees. 

Chief executive Kath Dalmeny said: “Millions of British consumers have already spoken; they do not want low standard food on their plates.

“We do not want chemical washes, hormones, excessive antibiotics and the most damaging pesticides to be used to produce what we eat. It needs to be good for us, and good for the planet, from farm to fork.

“Supermarkets need to reassure consumers that they’re not putting our health and the welfare of animals and the planet at risk with any of their ingredients or products.”

Truss said the new committee would focus on food standards, consumer and developing countries’ interests, how to advance standards around the world, and developing export opportunities for UK businesses.

In a letter to the National Farmers’ Union president, she said: “I wholeheartedly agree that any trade deal the UK strikes must be fair and reciprocal to our farmers, and must not compromise on our high standards of food safety and animal welfare.

“I have been very clear on both these points and will continue to fight for the interests of our farming industry in any and all trade agreements we negotiate.

“As you know this is the first time in over 40 years that the UK has pursued its own trade policy and I do recognise the importance of close engagement with the industry and a clear ‘roadmap’, as you put it, to help inform agricultural trade policy and apply appropriate safeguards in UK free trade deals.

“I am pleased therefore, on behalf of the government, to agree in principle to the establishment of a trade and agriculture commission under department for international trade auspices, subject to agreement on terms of reference.”