The DUP and Alliance are at each other's throats over claims of a unionist pact in next year's general election.
East Belfast Alliance MP Naomi Long and the DUP's Gavin Robinson engaged in a bitter row over claims he courted other unionists to help remove Mrs Long from her seat.
Mrs Long spoke of "dirty tricks" while Mr Robinson said the allegations were "despicable" and said "shame on her".
The angry exchange broke out after the DUP's vote share dropped by more than 4% and led to speculation about a potential DUP/UUP pact for next year's Westminster elections to help unseat the Alliance Party in East Belfast.
Senior DUP figures suggested the deal would involve the UUP opting out of East Belfast – where DUP leader Peter Robinson lost to Mrs Long in 2011 – in return for a free run in South Belfast, where the sitting MP is the SDLP's Alasdair McDonnell.
Mrs Long insisted she would hold on to the seat in a "fair fight" and hit out at any attempt to oust her by "dirty tricks", but the DUP's Gavin Robinson demanded that she apologise.
Mrs Long claimed that in his acceptance speech after retaining his council seat, Mr Robinson appealed to the PUP and TUV and other unionists to work together to win back the East Belfast seat for unionism.
"It was very grovelling stuff, he said they had just 50 weeks to do so," the incumbent MP added.
"He was appealing to the very people who have attacked the DUP repeatedly to join forces to unseat me.
"They have known since I won that it would take a pact to do so.
However, Mr Robinson accused Mrs Long of an embittered attack and putting words in his mouth and added: "In recent weeks I have been physically and verbally attacked by thuggish elements in East Belfast.
"I've stood against those who support drug dealers and other criminals.
"For Naomi Long to suggest that I was, at the same time, trying to hatch some deal with such people is despicable.
"Whilst Mrs Long took time on the airwaves to attack my character, I note she did not condemn those who physically attacked me. Rather, she chose to exploit the attacks for cheap party political advantage.
"When Naomi Long and her office were under threat and attack I condemned those responsible.
"When I am threatened and attacked, Naomi Long seeks to exploit it for party purposes. Shame on her."
Mrs Long said she had not been aware Mr Robinson had been attacked recently but said she condemned all acts of violence and intimidation – while the DUP had been less than consistent.
North Antrim MP Ian Paisley jnr hinted that discussions had also involved Fermanagh and South Tyrone but refused to name the UUP member he said was involved in meetings when challenged to do so by Ulster Unionist leader Mike Nesbitt.
Mr Nesbitt insisted there are no ongoing talks and argued the "political dynamic" in east Belfast has changed with the council successes of veteran Jim Rodgers and Sonia Copeland, the wife of MLA Michael Copeland.
He added yesterday: "If there have been conversations in recent months, they were at that exact level – tea-room chat between staffers. I state categorically, there has been no negotiation.
"What the DUP tried to do in the wake of the local government elections was to deflect attention from their worst performance in recent years.
"Simply, the DUP have peaked, the Ulster Unionists have bottomed out and are back on the rise, and the DUP couldn't handle it."
However, Mr Robinson warned that unionists could lose both Westminster and Assembly seats unless they adopt a "more coherent approach".
The DUP remained the biggest party in local government with 130 seats, Sinn Fein on 105, UUP 88, SDLP 66 and Alliance 32 – with the remaining 41 seats going to the smaller parties and independents, including Jim Allister's Traditional Unionist Voice which took 13 seats.