Ulster Unionist Party leader Mike Nesbitt today accuses First Minister Arlene Foster of "turning a blind eye" to Sinn Fein while attacking other unionists.
In a blistering attack on his party's main rivals, the UUP leader said: "The DUP's whole being is about trying to destroy other unionist parties."
Writing exclusively for the Belfast Telegraph, Mr Nesbitt also pointed out Mrs Foster had admitted the Opposition at Stormont had forced Executive parties to "up their game".
He also revealed that former First Minister Peter Robinson allowed him access to the DUP "engine room", which was populated by special advisers rather than elected representatives.
His counter-attack came after a no-holds-barred broadside by Mrs Foster, who argued there was an increasing impression that the UUP had "lost its way".
In a set-piece address at the weekend, she said the UUP had been reduced to the worst results in its history in the May Assembly elections and did not appear to have any core beliefs.
"A significant number of UUP members have approached me in the last few months wanting to join with us," Mrs Foster added.
Then at a breakfast-time event at the Conservative annual conference in Birmingham on Tuesday, she said she would support formalising an Opposition at Stormont if the current arrangements prove successful.
"Though the Opposition has been taking its time to get up and running, government is undoubtedly being forced to up its game and perform better under the new arrangements," the First Minister explained.
However, Mr Nesbitt hit back: "They can stick their heads in the sand over Nama or hope that someone else pulls a rabbit out of the hat to deal with Brexit, but that won't stop us raising these issues, no matter how uncomfortable it makes things for the DUP/Sinn Fein Executive, which is engulfed by paranoia and control freakery.
"Only today, First Minister Foster has openly admitted that the formation of the Official Opposition has forced the Executive to try to up its game. We will make sure that Mrs Foster and her DUP/Sinn Fein Executive colleagues are kept on their toes. That's a promise."
Mr Nesbitt's comments came as it emerged the UUP and SDLP - Stormont's two main Opposition parties - are to demand that ministers intervene over Northern Ireland's health service and housing crises.
The next Opposition day in the Assembly later this month is to focus on the "unprecedented crisis engulfing" the NHS in the province and demand action from the Health Minister, Michelle O'Neill.
With 392,000 people now on waiting lists, the Sinn Fein Minister is being urged "to ensure patient safety is not further compromised" in an Ulster Unionist motion.
DUP Communities Minister Paul Givan will also be urged to commit to building 8,000 new houses to tackle growing housing waiting lists and homelessness.
An SDLP motion will say there are 37,347 households on the social housing waiting list, "of which 22,986 were deemed to be in housing stress and 15,474 were deemed to be statutorily homeless".