Some British food, including beef and lamb, would become "uncompetitive overnight" on foreign markets in the event of a no-deal Brexit, food industry representatives told lawmakers on Monday.
Crashing out of the EU without an agreement would leave producers with customs arrangements set by the World Trade Organization - where agricultural tariffs could top 50 percent, Sarah Baker, from the Agriculture and Horticulture Development Board, said.
The United Kingdom is due to leave the European Union on March 29, but there is no full exit deal and Britain's ruling party is split over Prime Minister Theresa May's "Chequers" plan to largely accept a "common rulebook" over goods.
That "Chequers" plan was the least bad option for Britain, industry officials told the UK parliament's Scottish Affairs Committee.
A failure of that plan, and a subsequent departure without an agreement, would leave producers facing heavy tariffs, they added.
"Agriculture is probably the most heavily-protected (sector) and therefore tariffs of above 50 percent are not unusual (under WTO terms)," Baker said. Products including beef, lamb and seed potatoes "would just become uncompetitive overnight," she added.
The case was particularly pressing in Scotland where agriculture made up a bigger share of the economy than the UK as a whole, she told Reuters after the hearing.
Other products would take time to establish a strong presence outside Europe, said other officials.
"We are ambitious for markets beyond Europe, but at the moment Europe is the ball game," said James Withers, Chief Executive of Scotland Food & Drink.
Supporters of Brexit say it will let Britain sign its own trade deals with Europe and beyond, while freeing it from what they see as stifling Brussels regulation.
They have also said there is no need to leave without a deal and have accused opponents of Brexit of scare-mongering.
Steve Baker, a former junior Brexit minister who resigned over May's Chequers proposals in July, told the Press Association on Monday he thought May should seek a Free Trade Agreement under the terms placed on the table by European Council president Donald Tusk in March.