Abandoning mandatory coalition between unionists and nationalists in Northern Ireland would fundamentally change the Good Friday Agreement, the SDLP warned.
The 1998 accord which largely ended republican and loyalist violence represents peace and the first proper accommodation between Ireland and Britain for hundreds of years, the nationalist party’s leader Colum Eastwood told MPs.
The British and Irish Governments have said they are committed to the deal which enshrined Stormont devolution amid recent suggestions it could be revised.
Mr Eastwood said: “That is not tinkering, getting rid of mandatory coalition is not tinkering.
“It is a fundamental change in what the Good Friday Agreement represents.
“What the Good Friday Agreement at its very core represents is peace, it is the first time we have properly had an accommodation between the peoples of this island and the peoples of our island in many a hundred years.”
He was giving evidence to the Northern Ireland Affairs Committee at Westminster.
Mr Eastwood said those who criticised the Agreement were not recognising that the people of Ireland had voted for it and it was not going away.
“We cannot do anything that would undermine the progress.”
He acknowledged the Agreement was “imperfect, awkward and difficult”.
“It is what we have and we should not forget what got us here, we should not have such short memories to think that it is easy just to rip it apart and it won’t have any effect.”
A loyalist who helped organise 2016 protests against changes to the flying of the Union flag at Belfast City Hall also appeared before the committee.
Jamie Bryson said: “The Belfast Agreement was a surrender to IRA terrorism to stop them bombing England and that is the truth.
“It is a moral stain on the British Parliament.”
Mr Bryson said he had been urging a return to direct rule for years.
He earned 167 votes in 2011 local council elections and 350 in another poll and was appearing before the committee in his role with the Unionist Voice Policy Studies.
Unionist Voice describes itself as offering a platform to otherwise voiceless sections of the community and an avenue to report events from within the Unionist community in a fair and impartial manner.
It has been characterised as a loyalist blog.
Mr Bryson has more than 14,000 followers of his Twitter account.
Cross-community Alliance leader Naomi Long pulled out of giving evidence to the committee in protest at its decision to hear his evidence.
Former Sinn Fein Assembly member Daithi McKay resigned in 2016 over claims he coached Mr Bryson before an appearance at his Stormont committee to give evidence about a controversial property deal.
Mr Bryson is an opponent of the DUP and its former leader Peter Robinson.