Sinn Fein must "get real" about the existence of the IRA or face the political consequences, Mike Nesbitt has warned.
In a hard-hitting speech delivered to the Ulster Unionist Party conference, the UUP leader said his decision to walk out of the power-sharing Stormont Executive had been "vindicated" and that Sinn Fein's denials had worn a hole in the fabric of devolution.
Mr Nesbitt brandished a Guinness Book of Records saying he had nominated the DUP as the party with the most ministerial resignations. He will produce an alternative Programme for Government in next May's Assembly election.
First Minister Peter Robinson stood aside from his duties last month after Bobby Storey of Sinn Fein was questioned about a murder. Three of the party's four other Executive ministers also resigned, but were reappointed for a few hours each week to prevent political rivals getting the posts.
Mr Storey was released without charge and the DUP ministers took their seats again last week after a British Government report found the IRA still existed. Mr Nesbitt also withdrew his only minister, Danny Kennedy, and is staying out of the Executive.
Mr Nesbitt said "the current talks represent a unique opportunity for a collective political will to be expressed that we want to rid Northern Ireland, and the island of Ireland, of paramilitarism and the criminality once and for all.
"But we will not be standing shoulder to shoulder against paramilitarism with a political party that insists the IRA no longer exists. It looks like the DUP will. What a pity they won't stand strong with us. It's not the first time we have had to do the difficult thing on our own," he stated.
He warned: "Sinn Féin will defend the indefensible by saying the IRA are butterflies who have flown away. I make this offer to them. Get real. We want to respect your political mandate. Make it possible for us. Get on the same page as the rest of the world - or accept the consequences."
Elsewhere he signalled a softening on same-sex marriage, suggesting he did not support it personally but that it was inevitable. He said "to those of us who cannot bring ourselves to support same-sex marriage, I say this: we are on the wrong side of history. There is a new generation coming and they simply do not understand why there is a problem."
Mr Nesbitt believes it will be enforced by the courts, although there is a Stormont vote on the issue on November 2.
The main focus of his keynote address was on taking on the DUP and Sinn Fein through launching an alternative Programme for Government.
He promised it would "include the creation of a mental health champion. We have a number of champions... the one in four who suffer mental health issues deserve their own champion to promote and advance their cause".
It will oppose sectarian division. "Our alternative Programme for Government will shred the old binary approach, that insists everyone and everything has to be labelled orange or green and the money follows the labels... this party must be the defenders of our minority communities - zero tolerance for hate crime - total commitment to educating our children about tolerance and diversity."
He explained: "Identity is much more complex than a binary carve-up. I am a unionist leader, but I am not an Orangeman. I'm unionist, but in one very specific regard, I am as Irish as Caitriona Ruane. We have both represented Ireland at sport. It was clearly a long time ago for me!"