Doubts over access to export markets could have devastating consequences for the lamb industry in particular, which looked overseas for nearly a third of its income, the National Farmers’ Union said.
Its president, Minette Batters, clashed with the Environment Secretary, Michael Gove, as two conferences on farming got under way in Oxford.
In a no-deal scenario, exports to the EU from Britain could be subjected to huge tariffs, with beef potentially seeing a duty of 65 per cent, and lamb 46 per cent – potentially pushing up the costs for businesses.
Imports could also be affected, with possible delays at ports of essential items such as veterinary medicines, fertilisers, feed and machinery parts.
To avoid food price rises as a result of a no-deal Brexit, the Government could unilaterally lower import tariffs.
This would open the UK to goods that are not produced to the high standards of food safety, animal welfare and environmental protection met by British farmers, the NFU said.
Ms Batters said: “There have been enough warm words and comfort to us as farmers but now is time for decisions from the Government about how it will secure the nation’s food supply.
“We are less than 90 days away from Brexit and there is still enormous uncertainty about the future and how domestic food production fits into that.
“When I speak to people about food, they do recognise the importance of our sector, to our economy, to our environment and to our food security.
“Food is one of the fundamentals of life. Its importance cannot be understated. A Government that fails to deliver a Brexit that gets this right will fail us all.
“It is crucial that Government engages with our industry to deliver a sustainable, competitive and profitable British farming sector for generations to come.”
Mr Gove told the Oxford Farming Conference that leaving the EU without a deal would put at risk “real gains” from Brexit such as boosting productivity in farming and protecting the environment.
Although a nation as adaptable, resilient and creative as the UK would flourish over time even without a deal, he said, “the turbulence which will be generated by our departure without a deal would be considerable”.
Mr Gove added: “It would hit worst those who are our smaller farmers and farm businesses.”
He said tariffs, border checks and labour pressures would all add to costs for food producers.
“Nobody can be blithe or blasé about the real impacts on food producers in this country of leaving without the deal,” he told the conference.
Mr Gove played down the risk of consumers facing shortages on supermarket shelves.
“We are doing everything possible to ensure that when it comes imports both of food and veterinary medicines that we can maintain continuity as effectively as possible,” he said.
“There shouldn’t be a problem with food coming into the country.”