A former Tory MEP has said Brexit will lead to “certain meltdown” in rural Scotland with cheap food imports pushing farmers out of business.
Struan Stevenson predicted that markets will decline sharply as competition from outside the EU expands.
NFU Scotland said it was an “extremist” view but that it still had to be taken on board.
UK environment secretary Michael Gove has insisted that leaving the EU would be an “opportunity” for farmers.
Writing to The Herald newspaper, Mr Stevenson, who stood down from the European Parliament in 2014, said: “Farmers and landowners, far from benefiting from new worldwide trade deals promised by the arch-Brexiters, will see their markets decline sharply as competition from cheap imports expands. Subsidies will disappear. Land values will collapse.
“Most farmers have thin margins, if they have any margins at all. The European Commission estimates that land prices would fall by 30% if farm subsidies were totally abolished in the UK and they would fall sharply if subsidies were reduced.
“For farmers who have taken out bank loans against the value of their land, a loss of value would be fatal.”
He pointed out that many UK farmers currently receive 60% of their income from EU subsidies, and urged the government to commit to maintaining subsidies beyond 2020.
He added: “The Brexiters also claim that the EU’s protectionist policies discriminate against cheap food imports and force up food prices for British consumers. In other words, they want cheaper food following Brexit.
“That means throwing open UK markets to cheap food from Africa, Australia, North America, Brazil and Argentina, causing chaos for UK farm gate prices, a further fall in land values and widespread bankruptcies.”
Mr Stevenson was a Scottish Conservative MEP from 1999 to 2014.
In his article, he argued that a deal between the UK and the US could lead to hormone-treated beef and chlorine-washed chicken being imported – both of which are banned under EU regulations.
NFU Scotland has been lobbying for a more simplified support structure to replace Single Farm Payments post-Brexit and has emphasised their importance to the sector.
But president Andrew McCornick told BBC Scotland: “He is being a bit extremist. What we are asking for is a transition. We have built up a high standard in Scotland and that standard needs to be maintained.
“Michael Gove has already stated he would not allow substandard products to come in and flood our market and we would consider (chlorine-washed chicken)substandard.”
A spokeswoman for the UK government’s Department for Environment, Food and Rural Affairs (Defra) said it was committed to continuing the same cash total in funds for farm support until the end of this Parliament.
She added: “We are absolutely determined to get a good Brexit deal for Britain.
“Leaving the EU provides us with a golden opportunity to better support our farmers to grow more, sell more and export more great British food, while continuing tariff-free trade for all our goods.
“We have also been clear we will in no way dilute our high quality environmental and animal welfare standards.”
North-East SNP MSP, Stewart Stevenson, said: “More and more senior figures are condemning the UK government’s failing Brexit approach and their desperation to broker trade deals with new partners that will have devastating consequences for Scotland’s fishing and farming industries – sectors which pride themselves on high-quality produce.
“But the Tories in power are still blind to the dangers, unable to think outside their narrow party interests and face up to the facts about a hard Brexit and what it will mean for our rural industries.
“The Tory government’s Brexit plans continue to present a huge threat to jobs, investment and living standards in Scotland and Ruth Davidson and the Scottish Tories’ fulsome embrace of this prospect is truly shameless.”