UUP MLA Doug Beattie has accused the DUP of trying to wash their hands of the Stormont House Agreement after calls from the party to revisit Troubles legacy provisions.
He made the comments after it emerged that DUP leader Arlene Foster has written to Secretary of State Julian Smith to express her concerns around certain provisions in the 2014 agreement, including the establishment of a Historical Investigations Unit (HIU) to re-examine unsolved murders.
As part of the New Decade, New Approach deal the Government has promised to table legislation to implement the legacy provisions, agreed at Stormont House in 2014, within 100 days.
Mrs Foster has also expressed a desire to amend the definition of a Troubles victim to a "person who was killed or injured through no fault of their own".
Mr Beattie and his party have opposed the legacy mechanisms and repeatedly called for them to be dropped.
The UUP justice spokesperson said that the DUP had expressed support for an investigations unit in their 2019 General Election manifesto and in response to the public consultation on legacy.
"The DUP set us on this path at Stormont House and now it has left us for Westminster they are trying to distance themselves from what they created," he said.
"It is positive that they are now seeing what I have been saying for the past few years, but is it too late? Is it unstoppable?"
Mr Beattie accused the DUP leader of making "glib statements" around the HIU and challenged Mrs Foster to categorically state that her party cannot support it.
He said that the DUP had known for years what was in the agreement and now that it was getting down to the wire they had changed tact.
"It's not just myself, the Police Federation and retired RUC officers have spoken out against the HIU. It's absolutely disgraceful," the Upper Bann MLA said.
"I'm not against the HIU because I am a unionist or former soldier, I do not support an amnesty. If introduced it will stop thousands of people from getting justice and how can anyone support that.
"If you change the HIU in the way that's needed it will no longer resemble what was created in the first place."
The UUP MLA said he believed the DUP may have changed their position due to pressure from the Police Federation, Orange Order and other policing groups.
"This is the DUP preparing for the 2022 Stormont election. They are dancing on the head of a pin. The HIU is the brainchild of the DUP," Mr Beattie said.
Sinn Fein's Michelle O'Neill rejected calls the from Mrs Foster to revisit the legacy provisions.
The party's vice-president said that the agreement must be implemented in full and pointed out that the DUP signed up to the agreement, alongside the UK and Irish Governments and other political parties.
In her letter Mrs Foster wrote that in the five years since the agreement was signed "things have evolved" and that "the issues must now be addressed in light of the passage of time"
She said that the concerns must be addressed before any legislation could be introduced.
The DUP leader expressed concern that the HIU would "focus more on state-related deaths" and said that many victims and survivors were "losing confidence or not being supportive of the proposal".
Mrs Foster also questioned the HIU investigating "non-criminal police misconduct", saying it cannot under law reach any meaningful outcome.
"There are a range of additional issues that I would wish to raise with you, on how we can build on, improve and address issues since the high-level agreement on legacy as set out in the Stormont House Agreement," the First Minister wrote.
Mrs Foster called on the Secretary of State to begin discussions as soon as possible as the 100 day deadline "does not give much time for what will need to be substantive discussions on the way forward".
She outlined her reasoning in an interview on Sophy Ridge on Sunday.
"I think we need to revisit the Stormont House Agreement because what is being proposed, and we made this clear in our consultation response at the time, is not acceptable to the victims' groups here and it's certainly not acceptable to us," the Fermanagh and South Tyrone MLA said.
"There is a need to deal with these issues but it has to be done in a way that recognises fairness and recognises what actually happened here in Northern Ireland over 35-40 years, the fact that we did have a terrorist campaign and there were so many innocent victims as a result of that."
Mrs Foster also called on Prime Minister Boris Johnson to protect members of the security forces based in Northern Ireland from "vexatious claims" alongside their mainland counterparts.
Speaking following the interview, deputy First Minister Ms O'Neill said the Stormont House deal could not be "cherry picked by the British Government or the DUP".
She said that she had requested an urgent meeting with the Secretary of State to discuss legislation on victims persons and other legacy issues.
“The British government has been resiling from the commitments it made at Stormont House and has delayed implementing the agreement for more than five years, further delaying truth and justice to the victims and their families," the Mid Ulster MLA said.
“The British government needs to implement its commitments in full in a human rights compliant manner.”
The DUP has been contacted in relation to Mr Beattie's comments.