A by-election will be held in Craigavon after councillors failed to agree on a replacement for a representative who resigned earlier this year.
Traditional Unionist Mark Russell has stepped down from his Lurgan seat.
At a meeting on Monday, the Traditional Unionists failed to get their candidate seconded. The DUP and Ulster Unionists objected to each other's choices.
The by-election will take place early next year at an estimated cost of £25 - £30,000 to the ratepayer.
In August, it was announced that the TUV and DUP had agreed not to force by-elections in Craigavon or Ballymoney.
In Ballymoney, councillors accepted a proposal by the DUP to co-opt Robert Halliday into the seat.
Traditional Unionist Voice leader Jim Allister had previously indicated his party would force a by-election there to replace an outgoing DUP councillor.
Speaking after the deal in August, Mr Allister said the DUP had agreed not to oppose the co-option of a TUV replacement in Craigavon.
"In these circumstances, it was mutually advantageous to both parties to follow the course agreed and thereby the ratepayers of both Ballymoney and Craigavon can be saved expense."
However, no other councillor backed the nomination of the TUV's David Calvert by Independent Unionist David Jones.
"This TUV vacancy ought to have been filled by a TUV co-option, but the DUP welched on its agreement not to oppose a TUV co-option by proposing their own candidate," said Mr Allister on Monday.
"Our good faith in not forcing a by-election in Ballymoney has been shamelessly exploited."
The DUP, which did not back David Calvert, instead backed an independent candidate and rounded on the Ulster Unionists for not supporting him.
"We thought this well respected local man would have been broadly acceptable to most people on the council, indeed we had received some positive indications to this effect from Ulster Unionists," DUP councillor Stephen Moutray said.
"It is an utter disgrace that the UUP decided to oppose this compromise candidate.
"Had the DUP wanted to reclaim its seat we would have proposed a DUP member, but in the spirit of unionist cooperation we tried to reach an accommodation.
"We will not take any criticism from Jim Allister about the outcome.
"Our deal with Jim was that we would not oppose the co-option and we did not.
"However if his party's nominee was unable, on two occasions, to gain a seconder within the council he can hardly blame the DUP - he should be asking why Mr Calvert was unable to gain that support."
The SDLP's Dolores Kelly said the election had been brought about "satisfy political egos".