The four main farm unions have written to MPs warning that a no-deal Brexit could threaten the "safety, choice and affordability" of food for consumers.
The Ulster Farmers' Union along with its sister unions in Great Britain urged politicians to avoid leaving the EU without a deal.
They said it could have "severe impacts" on farm businesses, the food industry and a "fragile" rural economy.
The impact of tariffs on agri-exports was cited by farm leaders as a threat.
The risk of cheap food imports and the availability of veterinary medicines were also mentioned.
They said their claims were "not scare stories, but real possibilities".
The leaders of the National Farmers Union, NFU Scotland and the Welsh union NFU Cymru joined with UFU president Ivor Ferguson in signing it.
It was sent to MPs as they began a week of debate on the withdrawal agreement with the EU ahead of a vote in the Commons next Tuesday.
The farm leaders said they were committed to playing their part in managing Brexit in the best interests of farmers and UK public, but believed leaving without a deal would lead to the opposite outcome.
"We urge you in light of the central role Parliament will play in the coming days in resolving this impasse, to recognise the severe impact no-deal will have and to take all steps necessary to avoid such a departure coming to pass," the letter said.
'Historic political failure'
Agri-food exports to the EU could face tariffs of 27% on chicken, 46% on lamb and 65% on beef, they said.
Some 60% of UK food, feed and drink exports were to countries in the EU in 2017 according to their figures.
They also said in the event of a no-deal there would be pressure on the government to embrace a cheap food policy, opening UK markets to tariff-free imports which would undercut domestic production calling into question the viability of many farms.
The union leaders said Brexit would mean that for the first time in a generation politicians would be responsible for ensuring the UK was "properly fed".
They said if the outcome was a reliance on cheap imports produced to lower environmental and animal welfare standards it would represent an "historic political failure".
And they pointed out that while there appeared to a majority in the Commons against a no-deal Brexit, that with just three months to go, that was the default position in the absence of an agreement on an alternative.