A UK exit from the EU would create uncertainty about the open border between Northern Ireland and the Republic, an Irish diplomat has said.
Dan Mulhall was giving evidence on a so-called Brexit to the Northern Ireland Affairs Select Committee.
Mr Mulhall is the Irish ambassador in London.
He said a UK withdrawal could led to "some kind of customs controls which would be a negative development for both populations and economies."
Mr Mulhall told MPs that the Republic of Ireland wanted to "preserve the advantages" of its existing trade relationship with the UK, which he said was worth 65bn euros (£50.3m) last year and supported 200,000 jobs in each of the two countries.
"We have a very good set-up, never better than it is today, and we do not want to see anything cut across that," he said
He told MPs that Irish Prime Minister Enda Kenny does not believe a Brexit poses a threat to the peace process.
"He was talking about improved British-Irish relations which have developed. If you remove Northern Ireland from the EU it does make a difference," he said.