The speaker in the Northern Assembly has ruled that the proposed link up between the Ulster Unionist Party and the Progressive Unionist Party is contrary to the rules at Stormont.
The speaker, Eileen Bell, had sought legal advice on the issue following a challenge from Peter Robinson of the DUP.
If the move had received sanction, it would have given the Ulster Unionists an additional ministerial position in any power-sharing executive.
Earlier this year, the UUP leader, Sir Reg Empey, revealed plans for an alliance at Stormont between his party and the Progressive Unionist Party.
He justified the gamble by saying he hoped it would help to bring the UVF paramilitaries, linked to the PUP down a political road.
The move had other motives too. With the backing of the PUP's only Assembly member, David Ervine, the Ulster Unionists were in line to get an extra ministerial position in a power-sharing executive.
