Jeremy Corbyn will today call for the "spirit of the Good Friday Agreement" to be revived to protect peace in Northern Ireland after Brexit and get Stormont back up and running.
On a visit to Belfast on the eve of the 20th anniversary of the historic accord, the Labour leader will urge the Northern Irish parties, as well as Westminster, Ireland and Brussels, to play their part.
He will recommend the British-Irish Intergovernmental Conference - set up under the Good Friday Agreement - is reconvened to break the deadlock in Stormont.
And he will vow that Labour will never vote for a Brexit deal that could lead to a hard border on the island of Ireland.
Mr Corbyn's visit to Northern Ireland is his first since becoming Labour leader in 2015 and follows confirmation from his spokesman that he still supports a united Ireland.
DUP MP Ian Paisley has called on him to “condemn all IRA violence” before he speaks at Queen's University - where 29-year-old law lecturer Edgar Graham was killed by the terror group in 1983.
Mr Corbyn will say: “Look back at the sacrifice and courage shown at all levels of society that paved the way for something that had once seemed impossible. That was the spirit of the Good Friday Agreement.
“We all need that spirit again - Stormont and Westminster parties, the British and Irish Governments, business and unions, UK and EU negotiators - if we want to secure 20 more years of peace and greater prosperity for the many not the few.”
Northern Ireland's power-sharing executive has been on hold since January 2017 when the agreement broke down over a botched renewable heat scheme.
Mr Corbyn will argue: “If the current stalemate in Stormont cannot be sorted out in Belfast, I call on the UK government to reconvene the British-Irish Intergovernmental Conference.
"We must step up to find a creative solution, in the spirit of the Good Friday Agreement, that avoids a return to direct Westminster rule and lays the ground for further progress for all communities.”
And on Brexit he will say: “Labour will not support any Brexit deal that includes the return of a hard border to this island. But we are also clear there must be no border created in the Irish Sea either…
“Opposition to the idea of bringing back a hard border to this land isn’t just about avoiding paperwork or tariffs, important though that is, it’s about deep rooted cultural and community ties.
"An open border is a symbol of peace, two communities living and working together after years of conflict, communities who no longer feel that their traditions are under threat.”
Conservative chair Brandon Lewis said: “Labour are only interested in frustrating Brexit rather than making it a success for the whole United Kingdom.
“This Government remains resolute in our commitments to Northern Ireland, including upholding the Belfast Agreement.
“Labour on the other hand, say one thing in public but then in private say the Brexit risks to the Belfast Agreement were being played up."
Meanwhile, Mr Corbyn came under pressure from the DUP to "condemn all IRA violence" ahead of his visit.
Amid pressure over his past sympathies last year, Mr Corbyn said he believed the IRA were terrorists and its bombing campaign during the Troubles was wrong.
Asked if the Labour leader still supported a united Ireland, his spokesman said: "His view is on the record but in the context of the Good Friday Agreement and the new constitutional settlement in the north of Ireland the process by which such a thing could take place is clear and Jeremy supports that.
"It's a matter for the people of Northern Ireland and for the island of Ireland and it's something that can come about by the process laid out in the Good Friday Agreement.
"He over the years made his position clear that the majority of those people across the whole island of Ireland wanted to see that outcome - a united Ireland - but in the context of the Good Friday Agreement that can only come about through that constitutional process that's laid out in the Good Friday Agreement and Jeremy fully supports that."
DUP Brexit spokesman Sammy Wilson fumed: “He is taking a view that is contrary to that held by the vast majority of people in Northern Ireland and I suspect in the Irish Republic as well.
“People in Northern Ireland do not want to hear his anti-democratic views about defying the will of the people to break up the Union.
"They want to hear whether he has changed his views on his support for people who brought death and misery to Northern Ireland.”