The economic transformation of Irish border communities brought by the peace process has been put at risk by Brexit, MPs in England have been told.
The head of a business support organisation working in Counties Armagh and Down issued the stark warning as he gave evidence to the Northern Ireland Affairs Committee inquiry into the future of the Irish border when the UK leaves the EU.
Conor Patterson, chief executive of the Newry and Mourne Co-operative and Enterprise Agency, was appearing alongside other leading business representatives from the area.
All of them raised concerns about the impact of Brexit, highlighting issues around potential trading tariffs and restrictions on the movement of workers.
Mr Patterson told committee members that an unemployment rate of 30% in Newry at the height of the Troubles had dropped to below 3% this summer.
"Over the last 25 years this community has taken advantage of the dissolution of the border as a barrier to the movement of goods and people - growing world beating, locally-owned, innovative companies now employing thousands and making an enormous contribution to the Northern Ireland economy," he said.
He added: "Our concern is that transformation will be put at risk. We accept the result of the referendum but we wish the concerns of border communities to be taken into account by those negotiating both on behalf of the UK Government and the Government of the Irish Republic."
Mr Patterson said many local companies operated on a cross-border basis.
"Any disruption to trading modalities and especially freedom of movement of goods and people will badly affect these vital companies," he said.
He acknowledged the drop in sterling in the wake of the referendum had delivered a "windfall" for Newry retailers, but he said that would only continue if shoppers from the Republic of Ireland could travel north without restriction.
Michael Blaney, managing director at Newry-based insurance company Autoline, said his ability to attract talent from the Republic of Ireland could be undermined.
He also said the company's plans to expand its business across the border could be negatively affected.
Mr Blaney said the company suffered its first "Brexit casualty" shortly after the vote when one of its data scientists left.
"As soon as the Leave vote came through, he made plans to work in Dublin as he felt he had to remain within the EU," he said.
He added: "One of our major concerns is certainly how the flow of talent will be affected from markets across the border.
"Another of our concerns is that a significant part of our growth strategy was the natural step into the Republic given that two of our offices - Newry and Enniskillen - are so close to the border.
"We feel this would certainly become more difficult from a regulatory perspective if there was a hard Brexit with the passporting rights we currently have to trade in other EU jurisdictions being withdrawn perhaps."
Peter Conway, chief executive of Warrenpoint port, stressed how much the business relied on cross-border trade.
"One of the major concerns that we have is that 48% of the trade through the port emanates from or goes to the Republic of Ireland," he said.
Mr Conway, a member of the British Ports Association, said there were maritime concerns throughout the UK.
He said more than 50% of trade going through UK ports was linked to the EU.
"There are grave concerns in the industry," he added.