The US ambassador to Britain, Woody Johnson, has come under fire from a leading food critic, a farming union and trade justice campaigners over his push to open up the UK to American farmers post-Brexit.

Jay Rayner, the BBC presenter, Observer columnist and MasterChef critic, said the UK should tell Johnson where he can stick chlorinated chicken, the US’s preferred approach for protecting consumers from pathogens such as salmonella and campylobacter.

Writing in the Telegraph on Friday, Johnson attacked warnings that a post-Brexit trade deal would result in chlorine-washed chicken and hormone-pumped beef arriving on supermarket shelves.

“You have been presented with a false choice,” he wrote. “Either stick to EU directives, or find yourselves flooded with American food of the lowest quality. Inflammatory and misleading terms like ‘chlorinated chicken’ and ‘hormone beef’ are deployed to cast American farming in the worst possible light.

“It is time the myths are called out for what they really are. A smear campaign from people with their own protectionist agenda.”

Johnson said the process of using chlorine to wash chicken was the same as that used by EU farmers to treat their fruit and vegetables.

He described it as a “public safety no-brainer” and insisted it was the most effective and economical way of dealing with “potentially lethal” bacteria.

In a series of tweets, Rayner said “contrary to what he says, it’s risky stuff”. He quoted a study by the University of Southampton that found chlorine-washing does not kill pathogens. “The study was on salad vegetables but the lead scientist made clear to me that it applied to chicken as well,” Rayner tweeted.

“We know that Brexit is all downsides. It is often called an act of national self-harm, but letting in chlorine-washed chicken as Woody demands would be literally harmful to the health of the nation. It makes me sick, in so many bloody ways.”

Nick Dearden of the campaign group Global Justice Now, which has warned about the implications of the UK doing a trade deal with the US, said the EU had heavily restricted the use of chlorine in salad washing several years ago.

“The reason US farmers do it is because their animals are raised in such cramped conditions that the only way to stop them becoming diseased is to dunk them in chlorine,” he said. “It’s really an animal welfare issue here. If UK farmers want to compete against American imports they’ll have to lower standards or go out of business.”

The National Farmers’ Union (NFU) has also raised concerns over food safety and animal welfare standards when future trade deals are negotiated. “It is imperative that any future trade deals, including a possible deal with the US, do not allow the imports of food produced to lower standards than those required of British farmers,” said the NFU president, Minette Batters.

Jim Moseley, the CEO of Red Tractor Assurance, which oversees standards on many British farms, said: “Categorically, the UK’s food standards are now under threat from the commercial appetites of the United States food lobby. We urge the government not to sacrifice legislation which prevents these sort of products from being sold in the UK.

“British people deserve better than having their world-leading food standards sold out from underneath them. Our research shows that shoppers look for food that has been produced to the highest standards of food safety, animal welfare and traceability.

“A deal that allows illegal products to be brought into the UK, lets down the British public and undermines all the investment and efforts of British farmers. This cannot be the right thing to do.”

A Downing Street spokeswoman insisted: “We have always been very clear that we will not lower our food standards as part of a future trading agreement.”