Bord Bia has found that 40% of Irish food exporters it surveyed expect to export less as a result of the UK's decision to leave the European Union.
The UK takes 41% of Ireland's food and drink exports valued at €4.4 billion.
Bord Bia Chief Executive Aidan Cotter said his message to agri-food exporters is to stay calm and to build very close relationships with their customers in the UK.
He said staying close to the customers at this time is critical.
The Bord Bia Survey highlighted a lot of uncertainty among the food sector about the implications of the Brexit vote.
It found that 35% of the firms surveyed said they did not know what impact the United Kingdom leaving the EU would have on their business.
Earlier today, Bord Bia held a briefing in Dublin about Brexit that was attended by up to 200 stakeholders in the agri-food and grocery sectors.
James Walker, Chief Economist of IGD, an UK based grocery research organisation, said the UK grocery industry is still reeling from the Brexit vote.
He said the grocery sector is going to find itself in the absolute front line of the Brexit related changes.
Mr Walker also said that the expectation that most of the issues involved in UK leaving the EU could be decided in two years was staggeringly optimistic.
Brid Cannon, Assistant Secretary of the Department of Agriculture, said that all existing rules and procedures about trading with the UK will stay in place until two years after Article 50 is triggered and can be extended after that if all of the 27 EU member countries agree.
She warned however, that the uncertainty about trading relationships could go on for a few years before it is resolved.
The Chief Executive of Dawn Farm Foods Larry Murrin, said the UK is one of the most competitive food markets in the world and that the food industry is more important to Ireland than the automotive industry is to Germany.
He said that the 16% fall in the value of sterling since the start of this year is particularly challenging for newer food exporters to Britain who now effectively need a 16% price rise over the next six months to stand still in terms of profitability.
He said he is very concerned that the entire food sector should work together to manage what is going to be a very long period of uncertainty over the next two or three years.
The Irish Farmers Association National Treasurer Jer Bergin said the acid test for the Government, the EU, and industry as a whole is that if Brexit goes ahead that there should be no impact on family farm incomes.
He said that needs to be protected in every conceivable way. If that does not happen he said, all talks will have been a failure.
