Around two thirds of Northern Ireland's dairy business could be "killed off" post-Brexit, a committee of MPs has been warned.
Dr Mike Johnston, the Northern Ireland director of Dairy UK, said in order to keep the business alive, a period of continued access to the EU free trade agreements was vital.
It is estimated that two-thirds of UK dairy exports outside the EU are from Northern Ireland.
Dr Johnston warned that if measures are not put in place ahead of Brexit, export tariffs could double, effectively killing off the business.
He made his warnings during an evidence session of the Northern Ireland Affairs Committee on the future of the land border with the Republic of Ireland.
Dr Johnston, who was joined by other senior members of the region's agri-food industry, told MPs that the UK dairy industry exports to around 100 countries.
He said: "The total of UK Dairy exports outside the EU, about two-thirds, are from Northern Ireland. We have developed long-term relationships over the years and a lot of that is based on the 50-plus free trade agreements the EU has in place.
"What we would be looking for is continuity for a period of those agreements. If we had to switch overnight to world trade, to WTO (World Trade organisation) tariffs, it would just kill that business.
Referring to Thailand and Malaysia as examples of places where a lot of dairy is exported from Northern Ireland, he said the current export tariff is around 15%.
"If we had to move to a WTO tariff that would double, that would kill that business," he said. "What we need is a period of continued access to the EU free trade agreements for a period, but then we need the UK government to come in and start to negotiate and put in place free trade agreements that are specific to the UK to allow this business to continue."
Dr Johnston also warned that the industry is very dependent on free movement of milk, dairy products and people across the land border that has developed over the years through arrangements between the two jurisdictions.
He said if that free movement was stopped, there would be a situation in which dairy farmers could be forced out of business.
Barclay Bell, President of the Ulster Farmers Union, told the committee that the border "is a big, big issue when you consider the amount of cross-border trade".
"Anything that would complicate or hinder that trade certainly would be a big big challenge to the industry in Northern Ireland," he warned.
The agriculture and food processing sectors in Northern Ireland are a significant element of the region's local economy, collectively accounting for around 70,000 local jobs - based on 47,979 total farmers and workers and 23,557 food and drink processing workers.