Boris Johnson has hinted at allowing genetically modified food to be imported from the US after Brexit as he called for an end to “hysterical” fears about American products coming to the UK as part of a trade deal.

In a speech setting out his goals for trade after Brexit, the prime minister said the UK would not accept a “diminution of standards” on food hygiene or animal welfare as a result of a deal with the US.

He also said Britain would be “governed by science, not mumbo-jumbo” when looking at whether imported food was acceptable for consumption in the UK. Johnson criticised “America bashers” who take a “hysterical” attitude towards US food and view it as “inferior”.

He accepted that there was an argument against chlorinated chicken on animal welfare grounds, but his comments potentially open the door to other US practices coming to the UK, such as genetically modified food.

Play Video 1:38 'There is no need for a free trade agreement': Boris Johnson outlines UK stance after Brexit – video

“There are other issues where I think that I’ve heard a certain amount of hysterical … there is a sort of thing about as if American food was somehow inferior,” he said, speaking at the Old Royal Naval College in Greenwich.

“I look at the Americans, they look pretty well nourished to me. And I don’t hear any of these critics of American food coming back from the United States and complaining … So let’s take some of the paranoia out of this argument.”

Asked where the government stood on allowing genetically modified food into the UK after Brexit, Johnson’s official spokesman pointed to a speech from the summer in which the prime minister suggested it was time to “start now to liberate the UK’s extraordinary bioscience sector from anti-genetic modification rules”.

Neither No 10 nor Johnson would say which specific fears about US food were considered to be “mumbo-jumbo”, amid warnings from Labour that the US allows hormone-treated beef, rat hair in paprika, maggots in orange juice and rodent droppings in ginger.

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Some groups campaigning for UK food, environmental and animal welfare rules to be maintained after Brexit expressed alarm about Johnson’s comments, arguing that there could be lower standards if the prime minister’s promises were not enshrined in any laws or treaties.

Benjamin Halfpenny, from the Greener UK coalition of 13 environmental organisations, said: “Saying we’ll maintain the highest standards might also be considered mumbo-jumbo unless those commitments are put in law somewhere. Despite many opportunities and promises, the government hasn’t managed to do this yet.”

Nick Dearden, of Global Justice Now, also warned that the rhetoric of Johnson’s speech suggested he would be “intent on dancing to Donald Trump’s tune”.

“The only way Johnson can get his treasured trade deal with the US is to give away British standards and allow US multinationals to have a bonanza at the expense of people and the planet,” he said. “Beneath the usual bluster and bravado, we can see Johnson clearly rattled by the scale of opposition to a US trade deal – and so he should be. Johnson can tell those who oppose a US trade deal to ‘grow up’ all he likes. But what he labels ‘mumbo-jumbo’ is not anti-American at all, it is rather a deep-seated opposition to allowing the import of meat made in atrocious conditions, GM foods, higher medicine prices, cancer-causing chemicals, and handing over vast swathes of our society to big business.”

Labour called on Sunday for food and environmental protections to be enshrined in the new agriculture bill, warning that a failure to do so would be bad for consumers.

Luke Pollard, the shadow environment secretary, warned: “We won’t accept chlorinated chicken in our supermarkets or Boris Johnson selling out our animal welfare, food and environmental protections in a bid for a trade deal with Donald Trump.

“Promises that ministers have made to maintain standards aren’t worth anything until they are written into law, and unless they do so, we must surely conclude that they intend to break these promises during trade talks with the USA. “Labour will defend our environment, animal welfare and food standards, protect British consumers, and stand up for Britain’s farmers so they are not undercut by food produced to lower standards in the USA.”