Sir Jeffrey Donaldson last night urged new Ulster Unionist leader Robin Swann not to turn his back on unionist co-operation.
he Lagan Valley DUP MP was reacting to comments made by the North Antrim MLA, who ruled out a single unionist party during his speech at Saturday's AGM.
Writing exclusively for the Belfast Telegraph today, Mr Swann warns against "retreating to the trenches" following the disappointing election results.
He said: "A single unionist party would limit choice, stifle debate and quickly result in the depletion of unionist votes at the ballot box.
"That is the exact opposite of what we should be trying to achieve."
Last night Sir Jeffrey urged him to adopt a "more positive" approach on co-operation.
He said: "We don't have to have a single unionist party to have greater unionist unity.
"There is no doubt the two main unionist parties at present are able to appeal to a broader range of unionists, but as we have witnessed in the last elections, there is much greater scope through collaboration for us to maximise the number of unionist seats, whether at Westminster or the Assembly."
The Lagan Valley MP added: "We are keen to work with the UUP to ensure unionist representation is maximised. I hope Swann will adopt a more positive response to this than his predecessor, because Mike Nesbitt cost unionism dear."
Mr Swann was confirmed the new UUP leader on Saturday, following the resignation of Mr Nesbitt, who stood down on the day of the election count when the UUP went from 16 to 10 seats.
The TUV also urged more co-operation.
"There is certainly scope for greater co-operation between unionist parties," a spokesman said.
"The most obvious is to underscore the need to transfer across the unionist family.
"It is important to remember that reducing choice within the unionist family is unlikely to increase turnout.
"In that respect, Robin is correct."
In his speech to the party AGM, Mr Swann said: "Whilst we can cite our vote increasing to 103,314, we did not turn those extra votes into seats.
"Who knows, in the next few weeks we may be dusting down the posters and walking shoes again.
"But we must be able to channel the hurt and frustration we feel today into a positive energy that drives us forward and transfers into the strength and determination that will turn results around."
The new leader urged members to ask themselves: "Could I have done more? Was I fully committed?"
He added: "My challenge to you is to consider the part you will play in the next election."
Mr Swann said he believed the achievement of party newcomer John Stewart, who took a seat in East Antrim at the expense of Sinn Fein's Oliver McMullan, could be repeated elsewhere.
"It is this party that is most likely to, and is best placed to, take further seats off Sinn Fein, in places like West Tyrone and Newry and Armagh, and I have my eye on a few other seats as well," he added.
In his first address as leader, he also insisted the UUP will not join the Executive if political agreements are made solely for the benefit of the DUP and republicans.
"Whilst I do not rule out any possibilities, if these talks end up being talks and agreements made by two parties for the benefit of two parties, then I say let it be an Executive of two," he said.
"Standing at the great height of 5ft 3-ish, I have had my experiences of people trying to bully me and push me around… trust me, I have never been pushed around, nor do I intend starting to let people push me around, nor will I allow this party to be pushed around."
He also called for a funding boost to reduce health service waiting lists.
He said: "We have a daughter of six and a son of four - I want their experience of growing up in Northern Ireland to be very different from mine.
"I want them to grow up in a Northern Ireland at peace with itself, a Northern Ireland for all its people, a Northern Ireland which has a particular emphasis on the development of our children, which respects its elders and looks after the vulnerable."