A former Taoiseach says people on the island of Ireland are worried that a no-deal UK withdrawal from the EU would be the start of a "slippery slope" to a hard border, with checkpoints and troops.
Bertie Ahern also said Brexit has "raised tensions" and "complicated" progress towards a lasting peace.
One of the architects of the Good Friday Agreement, Mr Ahern voiced his concerns to a House of Commons committee on Wednesday morning.
Mr Ahern said: "Most people remember the border and remember sitting in long queues.
"They fear that any infrastructure at the border equals trouble, disagreement, Army, soldiers, police.
"Some of it might be exaggerated but there is that fear of the slippery slope. It is something that really worries people."
He also said the UK's 2016 vote to leave the EU was the reason why the Northern Irish institutions created by the Good Friday Agreement remain suspended after more than two years.
He said: "It's my view that if it wasn't for Brexit, the institutions in Northern Ireland would have been up and running a year ago.
"Brexit has stopped that. It wasn't the reason that brought them down, but it is the reason they are not back up."
He told MPs the Irish Government would not give up on the controversial backstop arrangements in Theresa May's Withdrawal Agreement, as they were viewed as "the only way of ensuring with certainty that we have a soft border".
He told the Commons Leaving the EU Committee: "There is no possibility of the Irish Government or the Irish people saying the backstop could be time-limited. There is no hope of that, I'm afraid."
Mr Ahern said continued membership of the EU for both Britain and Ireland was "taken as an absolute given" when he was negotiating the Good Friday Agreement with Tony Blair and was a "key element" on which the 1998 pact was built.
He denounced as "irresponsible" suggestions that Brexit should lead to an early poll on Irish reunification, saying this should wait until new arrangements have had time to bed in and the institutions are restored.
He told MPs: "The open and invisible border we have today is an achievement of the(peace) process and of our shared membership of the EU.”
While London, Dublin and Brussels had all voiced their determination not to erect physical infrastructure along the border in any circumstances, Mr Ahern said World Trade Organisation rules would force them to do so if the UK left without a deal.
Mr Ahern said a no-deal Brexit would be "devastating" for Ireland.
He said there was "almost total unanimity" in the Republic behind Mr Varadkar's insistence on the backstop, which is designed to keep the border open by keeping the UK in the EU's customs union and Northern Ireland observing certain single market rules until a wider trade deal is agreed.
"Brexit is disruptive, full stop," he said.
He concluded: "Whatever way you look at it - from the UK, from the Irish point of view, from the island of Ireland point of view, North and South - it badly affects us.
"There's very few plus sides – I can't think of any - in UK withdrawal from Europe."