Welsh farmers are too reliant on exporting to the EU, the international trade secretary has suggested.
With more than 90% of lamb exports destined for Europe, Liz Truss says "all the eggs are in one basket".
She said it showed the importance of opening up other markets around the world, something the UK government was "working hard" to achieve.
Ms Truss told the NFU Cymru conference that the country would "thrive" with or without an EU trade deal.
The union's president John Davies stressed that the continental market was of "supreme importance" to the Welsh farming industry.
"There is much to do and very little time if the UK government is to ensure a favourable trade deal with the EU.
"We cannot afford to face eye watering tariffs, even for a short time, on the bulk of our agri food exports."
Customs checks and other administrative burdens that could place "friction" on the movement of goods were also a cause for concern, he said.
Ms Truss insisted the government wanted "a good deal with the EU" and said it was in Europe's interest to agree one.
"In terms of agricultural produce, the EU exports three times as much to us as we do to them," she said.
"But in any negotiation you cannot just simply agree to the terms the other party is putting forward and you need to be prepared to walk away if the terms aren't favourable.
"My job as international trade secretary is to make sure there are plenty of other markets for Welsh producers to sell into."
There were "real benefits" in a recent trade deal struck with Japan, which would see the government launch a promotional drive involving Welsh lamb, with efforts also under way across the Middle East, USA, New Zealand and Australia.
Ms Truss said there was a growing demand for high quality protein across Asia which Wales was in a "very good position" to supply.
Welsh Secretary Simon Hart said it should not be underestimated "what a fantastic effort is being made" on behalf of food producers.
He said negotiations with the EU were in "all likelihood... going to go down to the wire - but that should not be a cause for panic."
Mr Hart claimed it was "the natural course" of negotiations all over the world and there was "no need for us to speculate now that it will end unhappily".