Britain has decided to "tear up" the Good Friday Agreement by going ahead with Brexit, a former Irish prime minister has said.
John Bruton argued the Brexit vote had "negated" the 1998 referendum held in NI and Ireland, which showed a majority in favour of the peace agreement.
He argued a no deal will lead to a hard border on the island.
Mr Bruton also said Sinn Féin's refusal to take their seats in Westminster was a "tragedy".
"Unfortunately in Ireland we had no say in this [Brexit] - the British people decided on this freely. In so doing, they effectively negated a referendum we had in Ireland," Mr Bruton told BBC's Today programme.
"Remember, we changed our constitution, took certain articles out of our constitution in return for an international commitment from Britain to the Belfast Agreement which guaranteed fair treatment of both communities in Northern Ireland, that neither community would be isolated.
"We changed our constitution to make that deal and Britain then comes along unilaterally and essentially decides to tear that up by proceeding with Brexit… and that's why we have insisted on a backstop to protect the Good Friday Agreement, so that Britain can't do that."
Mr Bruton served as Taoiseach between 1994 and 1997. He was later appointed as the EU's ambassador to the US between 2004 and 2009.
On Tuesday, members of parliament in the UK are expected to hold their vote on Prime Minister Theresa May's plan for the UK's withdrawal and future relationship with the European Union.
The key vote has been delayed from 11 December 2018.
The UK is scheduled to leave the EU on 29 March 2019.
"One suspects that those who object to the backstop are people who don't really ever expect there will be an acceptable agreement that would avoid a hard border in Ireland or between Ireland and Britain," added Mr Bruton.
On Sinn Féin's policy of abstention at Westminster, he said: "Ireland was partitioned in 1920 when Sinn Féin refused to take their seats after the 1918 election.
"Sinn Féin have refused to take their seats on this occasion and the most serious threats to the position of Northern nationalists are now about to be realised - with no Sinn Féin, no Northern nationalist voice there to argue a different case.
"I think it's a great shame, it's a tragedy."