THE SDLP has proposed a fresh anti-Brexit pact in three constituencies, despite the Greens rejecting a proposed alliance.
Party leader Colum Eastwood said it would support an independent, unaligned candidate in areas which voted to 'Remain' last year but have been represented by a Brexit-supporting MP.
It is understood the proposal would cover Fermanagh and South Tyrone, East Derry and North Belfast.
Green Party leader Steven Agnew said he would not be part of a pact because it could not strike a deal with others who do not share its "vision for a progressive society".
Sinn Féin northern leader Michelle O’Neill said she was disappointed and urged the Green Party to reconsider.
Mr Agnew said the SDLP's decision to reselect sitting South Belfast MP Mr McDonnell was a key factor in the party's decision.
He claimed fellow Green MLA Claire Bailey would have been a better unity candidate for Remainers and made clear that Mr McDonnell's pro-life stance on abortion meant his party could not support the former GP.
The North Down MLA also said Sinn Féin's continued abstentionist policy at Westminster made a deal with the republican party unworkable.
He said he had sought a meeting with Alliance leader Naomi Long but she had refused to engage.
"I am disappointed that Alliance chose to spurn further co-operation among pro-Remain parties," Mr Agnew said.
However, both Ms O'Neill and Mr Eastwood have indicated that hopes of some kind of pact are not entirely dead.
Ms O'Neill urged those parties who had toyed with the idea of an anti-Brexit front to reconsider, while Mr Eastwood said the SDLP would "continue to explore how we can maximise the mandate delivered by the (EU) referendum".
The Sinn Féin Stormont leader said the failure to forge what she termed a "progressive alliance" could "gift seats to pro-Brexit, pro-Tory and anti-equality hardliners".
"I am disappointed that parties which are opposed to Brexit and Tory cuts and which are pro equality have not been able to agree a progressive alliance to contest the Westminster election," she said.
Ms O'Neill urged her party's potential partners to reconsider, saying Sinn Féin was committed to maximising the anti-Brexit vote in June's poll.
Mr Eastwood said the collapse of the exploratory talks was a "deep disappointment".
He criticised Alliance for rejecting the pact proposal.
"It is to their shame that they then attempted to sectarianise the proposal," the SDLP leader said.
The Foyle MLA said the Greens' proposal that pro-Remainer Mr McDonnell stand aside was "simply a non-runner".
However, Mr Eastwood said the SDLP was floating a compromise idea that would see unaligned pro-EU candidates stand in North Belfast, East Derry and Fermanagh-South Tyrone.
"As an attempt at compromise, the SDLP has offered to support independent, non-party aligned, pro-Europe candidates in key battleground constituencies where the electorate voted to remain but the MP voted for Brexit," he said.
"Our sole intention has been to maximise the mandate delivered by the electorate in the referendum last year – it is a democratic travesty that despite 56 per cent of people voting to remain, only 22 per cent of Northern Ireland’s MPs voted against Brexit."
The collapse of the proposed anti-Brexit axis led Alliance to label the Greens "tainted", for what a statement issued in the name of a spokesperson claimed was a "seeming desire to turn several constituencies into a sectarian headcount".
"The Green Party have belatedly recognised their own naivety in even considering pacts, and have become more desperate in doing in the past few days," the statement said.
"To think Alasdair McDonnell, a socially conservative incumbent SDLP MP, would step aside in South Belfast or Sinn Fein would renounce their long-standing and counterproductive abstentionist policy was never realistic."
Meanwhile, there has been no progress publicly in efforts by the DUP and Ulster Unionists to forge a potential link-up ahead of June 8.
DUP leader Arlene Foster has already said her party will not stand against the UUP's Tom Elliott in Fermanagh and South Tyrone, while the Ulster Unionists will not be challenging Nigel Dodds in North Belfast.
However, talks between Mrs Foster and UUP leader Robin Swann on Monday have so far failed to result in a pact covering other constituencies including South Belfast.
