The UUP's only MP has said the decision by her party's assembly group to accept PUP leader David Ervine as a member has caused her "deep distress".
David Ervine joined the Ulster Unionist group at the NI assembly
North Down MP Sylvia Hermon told the House of Commons she was not party to the decision.
However, she said she had a lengthy discussion with her leader Sir Reg Empey on the matter on Tuesday night.
Lady Hermon said the move could prove worthwhile if it helped to deliver UVF decommissioning.
"If Sir Reg is able to establish now a policy to bring about loyalist decommissioning and prevent any more murders, any more Lisa Dorrians in my constituency, then that would be worthwhile.
"Apart from that I am deeply distressed by my colleagues' decision," she said.
Mr Ervine announced plans to join the UUP's assembly group ahead of Monday's inaugural meeting of the revived assembly.
The move means that the group grows to 25 assembly members.
It may also mean that there could 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.