Pushing Sinn Fein down the pecking order in the assembly was the "sole reason" why David Ervine agreed to join the Ulster Unionists' assembly group.
David Ervine joined the Ulster Unionist group at the NI assembly
The PUP leader said it "not only restores the majority balance in the executive but it pushes Sinn Fein from second pick to third pick".
The move would give the UUP an extra ministerial seat at Sinn Fein's expense if a power-sharing executive is formed.
Alliance will ask the IMC if the party could be sanctioned over UVF activity.
Alliance leader David Ford says he wants clarification from the Independent Monitoring Commission (IMC).
The ceasefire watchdog has imposed sanctions in the past on the PUP, because of its links to the loyalist paramilitary UVF.
Mr Ervine told the BBC's Talkback programme on Thursday that there was only one reason why he joined the UUP's assembly group.
"Simply, to move one seat to either the DUP or the Ulster Unionists would restore a majority balance on the executive," he said.
"That is the sole reason why I'm doing what I'm doing."
Meanwhile, UUP leader Sir Reg Empey has defended the decision to allow Mr Ervine to join his party's assembly group.
Sylvia Hermon, the party's only MP, had criticised the move.
"I believe we have done the right thing and in the long term the question is, do we bring paramilitarism from loyalists to an end?" he said.
On Wednesday, Lady Hermon said she had not been consulted about the move.
Lady Hermon also said the move had caused her "deep distress", however, she added that if it helped to deliver UVF decommissioning it could prove to be worthwhile.
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 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.