Four Labour assembly members have broken ranks to criticise their party's proposed coalition with Plaid Cymru.
Rhodri Morgan and Ieuan Wyn Jones want their parties' support
Torfaen's Lynne Neagle was first to speak publicly against the "One Wales" deal, which Plaid and Labour delegates will decide on this weekend.
Now her colleagues Karen Sinclair, Ann Jones and Irene James have said they too will oppose the agreement.
Labour MP Kim Howells is also said to have warned it would help "nationalists to the gates of independence."
After Ms Neagle wrote in the Western Mail that those against the ground-breaking alliance had been ignored, the Labour AMs for Clwyd South, Vale of Clwyd and Islwyn respectively issued a joint statement condemning a coalition.
They said: "It is our belief that the proposed 'One Wales' arrangement between Labour and Plaid Cymru will not provide for the people of Wales the stability it needs at this time."
The three AMs said they had made their decision "with regret, and after a great deal of soul-searching".
But they said: "The lack of any binding common philosophy between the two parties coupled with the different aspirations we have for the future of Wales would... make it very difficult for the coalition to withstand the rigours of a four-year assembly term."
They went on: "We have serious objections to a number of the Plaid Cymru-inspired policies included in this agreement and believe the document represents a significant departure from what people voted for in the original devolution settlement in 1997.
"In our view, the commitment to campaigning for a referendum (on turning the assembly into a Scottish-style parliament), even before the current new powers have had a chance to bed down, is undeliverable.
"It would lead to the focus being taken away from the real social justice issues that the people of Wales clearly want us to concern ourselves with over the next four years."
The One Wales agreement has received backing from both Labour and Plaid AMs and Labour's Welsh executive, as well as Plaid's national executive.
But an "overwhelming majority" of Welsh Labour MPs are said to have "serious concerns".
One, Pontypridd MP and Foreign Office minister Kim Howells, is reported by the Western Mail to have written to his constituency Labour party, urging the rejection of the deal.
According to the newspaper, Mr Howells wrote: "By inviting this coalition, we will be helping to deliver our communities into the hands of nationalist incompetents and separatists."
He also said: "It is ironic that the very same party that for so long held at bay the separatists and cultural and political nationalists is prepared, now, to provide for their former enemies an assembly vehicle that transports those same nationalists to the gates of independence."
Lynne Neagle says party members should be consulted
Labour failed to win a majority in the 3 May assembly election and has been running a minority administration at Cardiff Bay.
But a historic power-sharing deal with Plaid Cymru was agreed in principle by First Minister Rhodri Morgan and Plaid leader Ieuan Wyn Jones.
Plaid abandoned the idea of a "rainbow alliance" with fellow opposition parties, the Liberal Democrats and Conservatives, in favour of the move.
The deal now needs to be approved at a special Labour conference on Friday, then by a Plaid conference the following day.
The conferences will give grassroots supporters of both parties the opportunity to air their views.
In her Western Mail article, Ms Neagle, chair of the assembly group, expressed her concerns about the plans and about the way those opposed to the plans were being treated.
She said Labour members, councillors and MPs deserved to have their say on such a momentous decision, but they had been discounted and insulted for their views.
The AM wrote that Labour could and should continue in power as a minority government and that the One Wales plan was "a fundamental and detrimental change in direction for Welsh politics".
"If the idea of having Tory ministers is so repugnant to us in Welsh Labour, why are we prepared to govern with a party that would have inflicted that very curse on Wales had they not been let down by the Liberal Democrats?" she asks.