A veteran unionist figure has said that if Ireland was reunited by a narrow margin in a border poll then it would spark "civil war".

Lord Kilclooney, a former top UUP MP who now sits as a cross-bench peer in the House of Lords, was speaking to the News Letter after the taoiseach declared that he would not want Irish reunification to come about as a result of a mere 51%-to-49% margin.

Leo Varadkar dismissed the prospect of a united Ireland without significant majority support in Northern Ireland

The taoiseach's words – drawn from an interview on BBC Spotlight last night, and circulated publicly before broadcast – stirred a strong reaction from both the SDLP and Sinn Fein, who said his stance ran counter to the Good Friday Agreement.

Mr Varadkar said: "I wouldn't like us to get to the point whereby we are changing the constitutional position here in Northern Ireland on a 50%-plus-one basis.

"One of the best things about the Good Friday Agreement is that it did get very strong cross-border support – that's why there was a 70% vote for it. I don't think that there would be a 70 % vote for a united Ireland in the morning, for example, or anything remotely to that."

SDLP leader Colum Eastwood declared in a statement that the "principle of consent in the Good Friday Agreement cannot be undone".

Whilst he said he rejected the "numbers game", he concluded that the Good Friday Agreement "provides that a simple majority is the mechanism" for a united Ireland.

Sinn Fein MLA Conor Murphy said "the overwhelming majority of people on this island support Irish reunification".

He added: "If a simple majority vote in favour of reunification, both governments are then obliged to legislate for it. The Good Friday Agreement is the legal and internationally binding position."

In a 2015 interview, published in the News Letter, Lord Trimble – one of the key architects of the Good Friday Agreement – said he "knew that we needed over 70% [of voters to back the agreement] to be certain that we had a majority of unionists behind us... if we had fallen short of that we would have been in an enormously difficult situation".

Yesterday, Lord Kilclooney told the News Letter that polls do not show anything like 50% support for a united Ireland in the Province, so "to talk about more than 50% is of course, in my opinion, absolutely crazy".

He said the bulk of the Catholic community vote for nationalist parties for "reasons of community division and not because they actually want a united Ireland".

He added: "Assuming we were wrong on this, and there was a 50.1% in favour of a united Ireland, in no way would one dare have a united Ireland.

"Because the reality on the ground in Northern Ireland is there would be civil war.

"You cannot force Northern Ireland out of the UK by a 1% majority. Can you imagine the loyalists in Belfast taking it quietly? I couldn't.

"But anyway, I think these figures – 50.1% or 70% – are out of the question. In my opinion the cause of a united Ireland is fading within Northern Ireland."