The Ulster Volunteer Force (UVF) should "catch up" with republicans and address criminality and paramilitarism, NI Secretary Peter Hain has said.
NI Secretary Peter Hain urged the UVF to end paramilitarism
He was speaking after Ulster Unionists admitted Progressive Unionist leader David Ervine into their new group in the revived Stormont assembly.
The move angered some unionists because the PUP is linked to the UVF.
Mr Hain said the PUP leader's decision to link with the Ulster Unionists was "a surprise" but was their prerogative.
"It is a matter for them," he said.
"They can sit wherever they like and under which umbrella they like."
However, the NI Secretary said Sinn Fein was in a "much stronger position" to be on an executive, since the IRA had decommissioned.
"The UVF have not ended their paramilitary campaign, they are still involved in criminality in a big way and all of those things are things which the IRA have promised to deliver on and is delivering on," Mr Hain said.
"Therefore, Sinn Fein are in a much stronger position to claim a seat in a power-sharing executive since they have decommissioned, they have ended their paramilitary campaign and, according to the Independent Monitoring Commission, they are driving criminality out of their ranks.
"Now that is a big, big advance compared with where the UVF are.
"The UVF ought to catch up quickly."
Mr Ervine announced plans to join the Ulster Unionist party's assembly group ahead of Monday's inaugural meeting of the revived assembly.
The move means that the group now grows to 25 assembly members.
It will also mean that there will be a unionist majority in any future Stormont Executive and will give the Ulster Unionists an extra ministerial seat at the expense of Sinn Fein.
The UUP will also be called ahead of Sinn Fein in the assembly under its speaking rights.
Assembly speaker Eileen Bell has been asked by Ian Paisley's Democratic Unionists to check the legality of the move.