British farmers fear they could go out of business following a post-Brexit trade deal with the US, the National Farmers Union has told the BBC.
NFU president Minette Batters said US farms could "outcompete" UK producers if standards were changed.
Speaking on the BBC's Today programme, Ms Batters challenged the government to provide reassurance to farmers.
"Are we going to hold our nerve or are we going to be sacrificed?" she said.
Last week, the head of America's farming lobby argued the UK should accept US food standards.
A US-UK deal is being touted as a way to boost the UK economy after Brexit.
Ms Batters said farmers would like to see EU regulations enshrined in UK law after Brexit, but that it had "not been agreed" by the government.
"We need that assurance from the prime minister", she added.
Last week, Zippy Duvall, head of the US National Farm Bureau Federation, vowed to "make sure" that US food standards prevail in a trade pact.
Mr Duvall told the BBC he wanted to have "a conversation" about food standards given the concerns in the UK.
US practices that are banned by the EU include irradiation, the use of gamma radiation to eliminate micro-organisms, and the use of genetically modified crops.
One of the most controversial practices is washing chicken with chlorine to kill germs. Critics argue not that the wash itself is harmful, but that it permits poorer hygiene standards earlier in the production process.
Mr Duvall argued that fears over such practices were not "science-based".
But Ms Batters said that the US is not addressing animal welfare and environmental protection, areas she said were not subject to US regulation.
She argued that a solution would be for agriculture to be left out of an agreement.
"There is a huge difference in opinion in what's needed in a US-UK trade deal", Ms Batters said.
Mr Duvall, a poultry farmer based in Georgia, previously warned that leaving agriculture out of a deal would be considered a betrayal by US farmers.
"It would be turning their backs on rural America, and that's where a big part of the population lives."
Ms Batters, who runs a mixed farming business in Wiltshire, acknowledged that "agriculture is always the last chapter in any trade deal to be ratified" because of the compromises often involved.
UK farmers have recently raised concerns about a no-deal Brexit, which could lead to high tariffs on goods exported to the EU.
Lamb and live sheep exports could face tariffs of 45-50%, while some cuts of beef could see tariffs of more than 90%.
If European firms start having to pay more for UK meat, they could switch to suppliers in other countries.
Other so-called "non-tariff barriers", like extra veterinary and customs checks at the border, could also increase costs to farmers.
Farms also currently receive more than £3.5bn a year in EU subsidies under the Common Agricultural Policy.
Agricultural land covers 71% of Britain's total land area, amounting to 17.4 million hectares.
Agriculture's contribution to the national economy stands at less than 1%, according to 2018 figures.