Sinn Fein has promised to take legal action in its attempt to force Lisburn City Council to accept a formal method of power sharing.
Lisburn council's senior positions are all currently held by unionists
The council has put off a vote on the Sinn Fein proposal but the party said the issue must be dealt with as soon as possible.
Lisburn council's senior positions are all currently held by unionists and Sinn Fein said new procedures must be put in place to more accurately reflect the make-up of the authority.
Despite support from the SDLP and Alliance councillors, Sinn Fein was unable to push through its proposal at a meeting of the full council on Tuesday.
The proposal was referred back to a committee and will not be discussed by the council again until September at the earliest.
However, Sinn Fein Councillor Paul Butler said his party is not prepared to wait that long.
He said: "What we're going to do now is bring this to the courts.
"We want a judicial review of this decision, the decision to discriminate against nationalists.
"I think the courts of this land now need to find Lisburn council guilty of discrimination against nationalists and others. That's how serious this is."
Ulster Unionist Party Councillor and mayor Billy Bell accused Sinn Fein of seeking to score points on the issue.
The council rejected the proposal
He said: "If you look at the three previous mayors to me, for instance, two of them have been Catholic and one Protestant.
"Our record here on power sharing is as good as any other council in Northern Ireland."
Councillor Paul Porter, of the Democratic Unionist Party, said: "For the last 22-odd years, the DUP have had no positions on this council.
"It's amazing that we had no rumpus from Sinn Fein/IRA and other councillors in this chambers about the discrimination against our party."