Fears over chlorine-washed chicken and hormone-fed beef are “myths”, according to the US ambassador to the UK.
EU rules currently limit US exports of certain food products, including chicken and beef – but Mr Johnson wants that to change in the UK after Brexit.
Downing Street has repeatedly denied it will accept lower food standards.
A No 10 spokeswoman said: “We have always been very clear that we will not lower our food standards as part of a future trading agreement.”
Mr Johnson, however, described warnings over US farming practices as “inflammatory and misleading” smears from “people with their own protectionist agenda”.
He also said the EU’s “Museum of Agriculture” approach was not sustainable, adding: “American farmers are making a vital contribution to the rest of the world. Their efforts deserve to be recognised.
“Instead, they are being dismissed with misleading scare-stories which only tell you half the story.”
On chlorine-washed chicken, Mr Johnson said the process was the same as that used by EU farmers to treat their fruit and vegetables.
Describing it as a “public safety no-brainer”, he insisted it was the most effective and economical way of dealing with “potentially lethal” bacteria such as salmonella and campylobacter.
The US National Farmers’ Union has always maintained that its chicken and beef, which use processes banned by the EU, are “perfectly safe” and argues there has been a lot of “fear-mongering”.
However, its British counterpart, the National Farmers’ Union (NFU), said the UK government should not accept a US deal “which allows food to be imported into this country produced in ways which would be illegal here”.
The UK’s NFU also raised concerns over animal welfare standards when the US negotiating objectives were announced on Friday.
Amy Mount from Greener UK, an environmental lobby group, said: “This wish-list shows that a hard-Brexit pivot away from the EU in favour of the US would mean pressure to scrap important protections for our environment and food quality.
“Any future trade deals should reflect the high standards that the UK public both wants and expects.”
What is chlorine-washed chicken?
In the US, it is legal to wash chicken carcasses in strongly chlorinated water.
Producers argue that it stops the spread of microbial contamination from the bird’s digestive tract to the meat, a method approved by US regulators.
But the practice has been banned in the EU since 1997, where only washing with cold air or water is allowed.
The EU argues that chlorine washes could increase the risk of bacterial-based diseases such as salmonella on the grounds that dirty abattoirs with sloppy standards would rely on it as a decontaminant rather than making sure their basic hygiene protocols were up to scratch.
There are also concerns that such “washes” would be used by less scrupulous meat processing plants to increase the shelf-life of meat, making it appear fresher than it really is.