Creating a new speaker role at Stormont will poison the atmosphere in the Assembly and produce new inequalities, MLAs have warned.
The DUP and Sinn Fein's proposal to share the post of principal deputy speaker will further exclude the minor parties and send the wrong message to the public, the SDLP claimed.
The main parties in the power-sharing administration intend to promote Sinn Fein Deputy Speaker Francie Molloy to the new position before he swaps roles with current Speaker Willie Hay in three years.
Mr Molloy is one of three deputy speakers along with the SDLP's John Dallat and the UUP's Roy Beggs. The SDLP, Ulster Unionists and Jim Allister's TUV tabled a motion proposing that when the role is created it should be shared among the other deputies on a yearly rota.
Conall McDevitt, SDLP South Belfast representative, said: "I think it is critically important we do debate this because the impact of this is going to impact the culture of this house.
"The thing that concerns me most is that we are seeking to redefine equality in this house. Equality has gone from, we all come here and share the burden of responsibility in an equal way ... to Sinn Fein and the DUP are more equal than others."
While the Assembly has already voted to create the post, MLAs debated the procedural changes needed to introduce the role. Mr McDevitt's party colleague Albann Maginness stressed that a principal deputy speaker would have no greater legal status but warned that the DUP and Sinn Fein may work to give the office holder new powers.
Mr Allister said no one had yet been able to provide any reason why such a position was needed, while the UUP's Danny Kinahan said the DUP/Sinn Fein deal was "another nail in the coffin of democracy in Northern Ireland".
But the main parties rejected the arguments outright. Sinn Fein's Gerry Kelly stressed there would be no extra pay for the new role, while the DUP's Trevor Clarke pointed out that the move brought the Assembly into line with other legislatures such as Westminster.
The SDLP, UUP and TUV proposal was voted down by DUP and Sinn Fein members.