Jim Allister's TUV brokered a co-option deal with the DUP
The DUP and Traditional Unionist Voice have been accused of "breathtaking arrogance" after striking a deal to avoid two costly council by-elections.
The parties unexpectedly brokered an agreement to co-opt councillors in Ballymoney and Craigavon.
Sinn Fein said the TUV was preventing one of its Craigavon councillors being co-opted by threatening to force a by-election costing rate-payers £25,000.
The party has described the DUP and TUV agreement as "worthless".
Under council rules, if a party wants to replace a councillor it can do so without holding a by-election provided none of his or her colleagues from other parties object.
It had been expected on Monday night that the Traditional Unionists would force a by-election in Ballymoney where a DUP member was stepping down.
But then both parties confirmed they had reached an understanding which would allow the DUP to replace its councillor in Ballymoney without a by-election, whilst the DUP would not object to the Traditional Unionists replacing their councillor, Mark Russell, in Craigavon.
However, on Tuesday afternoon, Sinn Fein pointed out that their MLA John O'Dowd had wanted to stand down from Craigavon Council for some time, but understood that if he did, the TUV would trigger a by-election costing the ratepayers £25,000.
Sinn Fein wants an arrangement which will cover John O'Dowd too, but the Traditional Unionists said they would not seek or give republicans favours and will fight them in a by-election if necessary.