Mr Jeffrey Donaldson has rejected a call by Mr David Trimble to resign and then recontest his Westminster seat as UUP Assembly members prepare to meet privately to assess the difficult challenges ahead.
Recrimination over the decision of Mr Donaldson and fellow former UUP Assembly members Ms Arlene Foster and Ms Norah Beare to join the DUP continued yesterday with Mr Donaldson and Mr Trimble leading the angry exchanges.
The defections and the DUP overtaking the UUP in the Assembly elections triggered some speculation that there would be immediate pressure on Mr Trimble's leadership, but sources who attended a meeting of the UUP Assembly party at Stormont yesterday said the issue did not arise.
Instead it was agreed that later this month the party's Assembly grouping, now reduced from 27 to 24, would meet for two "away days" later this month to discuss its approach to the forthcoming review of the Belfast Agreement and consider its general future.
A senior party source denied that the focus of the two-day get-together would be on the fallout from the elections results and the Donaldson defections. "We are putting that business behind us. The meeting will be about the review, how we deal with the DUP and Sinn Féin in the discussions, what our Assembly party policy should be, and other similar matters," he said.
Meanwhile, at press conferences and in statements and over the airwaves, several UUP and DUP politicians verbally weighed into each other in the aftermath of Mr Donaldson, Ms Beare and Ms Foster switching to the DUP.
Mr Trimble said Mr Donaldson must decide whether to "do the honourable thing" and resign his Lagan Valley Westminster seat, won as an Ulster Unionist, and then contest it for the DUP.
Mr Trimble said the UUP was "inundated" with calls from people angry that the three defectors were elected to the Assembly as Ulster Unionists but had now changed sides to the DUP. He challenged Mr Donaldson to recontest his Westminster seat on the basis that on resignation or death Assembly seats are filled by co-option whereas Westminster seats require a by-election.
"A lot of people in Lagan Valley feel there should be a by-election. I think it is a matter for Mr Donaldson's honour and integrity. It is up to him to decide. It lies in his hands," said Mr Trimble.
Mr Donaldson said Mr Trimble should do the "honourable thing" by resigning as UUP leader.
He said a Westminster election was likely in 18 months when he would be happy to again face the electorate in Lagan Valley.
"Mr Trimble seems to ignore the fact that it was he and a small number of his supporters who orchestrated the move to have me expelled from the party. Is he suggesting that if I had been expelled there should have been a by-election?" asked Mr Donaldson.
The DUP deputy leader Mr Peter Robinson said Mr Trimble's "hypocrisy knew no bounds" while Mr Donaldson, Ms Foster and Ms Beare had acted with honesty and integrity.
"The fact that the unionist people couldn't quite be sure what David Trimble's position was, after having jettisoned election pledge after election pledge in the past, was a contributing factor to the UUP's defeat at the polls in November.
"David Trimble needn't tell me about let-down and deceived voters for he has been letting down and deceiving unionist voters for years," he said.
UUP Assembly member Dr Esmond Birnie said he would be ignoring the DUP's drive to entice more Ulster Unionists.
"A few UUP members may join the DUP in its flight from political reality but a mixture of bluster, barge and bombast does not work. Not now, not ever," Dr Birnie added.