A report confirming IRA decommissioning is complete has been given to the British and Irish governments at a meeting in Northern Ireland.
General John de Chastelain, head of the body overseeing the disarmament, is to give a news conference later with two independent church witnesses.
They are Catholic priest Father Alex Reid and ex-Methodist president the Reverend Harold Good.
The IRA announced an end to its armed campaign in July.
The republican organisation said it would follow a democratic path ending more than 30 years of violence.
Statements are also expected on Monday from the IRA, both governments and Sinn Fein President Gerry Adams.
DUP MP Nigel Dodds said: "What we have said all along about decommissioning in the post 9/11 world is this - that it is about building and instilling confidence in the unionist community.
"In order to do that, this was a position that all parties and all governments had agreed on - was that it was absolutely vital to have that visual aspect.
"That has now gone and it is going to be a lot harder, a lot more difficult and a lot more challenging to get people to accept that what was agreed last December has been set aside and now somehow we are asked to proceed as if that didn't matter."
IRA is expected to make a statement on its arms
Ulster Unionist leader Sir Reg Empey said General de Chastelain's report must be unambiguous.
"Obviously we need to get some greater sense of what has actually occurred instead of the general speaking in code," he said.
"One of the steps he could take is to give us an inventory of what has actually been decommissioned and disposed of."
SDLP leader Mark Durkan said the British and Irish governments must now move ahead with implementing the Good Friday Agreement.
"Trust has been damaged by the IRA's failure to decommission before now and Sinn Fein's arguments that the Agreement didn't even require it," he said.
"But the fact is that now it has happened and the evidence is there - if General de Chastelain's statement is clear and supporting evidence is clear - then we can take it to the full."
Sinn Fein's Alex Maskey said this would be a "historic day, not only for republicans but for everyone else in society".
"I think republicans will be saying today that the onus is now very clearly on the shoulders of the DUP and both governments," he said.
Alliance leader David Ford said he was "encouraged" by the news.
However, he said there were "crucial questions" for the republican movement, including whether or not they were prepared "to accept the legitimacy of the northern and southern states with respect to policing and criminal justice".
General de Chastelain, Andrew Sens and Tauno Nieminen - the commissioners of the Independent International Commission on Decommissioning - have been in Ireland overseeing the latest round of decommissioning since the beginning of September.
BBC NI security editor Brian Rowan said: "My understanding is that at the new conference, the general will tell us that the weapons and explosives put beyond use corresponds and matches the estimates he has been given by the security forces north and south of the border.
"We are not going to get an exact inventory. The IRA has something in the region of 1,000 rifles, 550 handguns, and two and a quarter tonnes of Semtex.
"He is going to be able to tell us that working on those estimates, he has now got the job of decommissioning done and it is now moving on to the next phase of his work, which is to concentrate on loyalist decommissioning."
Professor Paul Bew from Queen's University, Belfast, predicted that any move back to self-government would be a slow process.
General John de Chastelain expected to report on arms move
"There is a widespread assumption here that it will take time to convince the unionist people of Northern Ireland that Sinn Fein has become a party just like the others.
"We are into a period of several months, if not years, of political delay yet."
Speaking on Sunday, Northern Ireland Secretary Peter Hain said any move on IRA decommissioning must be credible enough to convince unionists.
"The people of Northern Ireland will want to see it actually implemented," he said.
He said once unionists knew decommissioning was credible and had been put in place, moves could be made towards restoring devolved government in Northern Ireland.