An "overwhelming majority" of Welsh Labour MPs are said to have "serious concerns" about the coalition deal between their party and Plaid Cymru.
Rhodri Morgan and Ieuan Wyn Jones want their parties' support
The deal to form a Labour-Plaid Welsh assembly government needs approval at special conferences on 6 and 7 July.
Many MPs are unhappy about plans to campaign for full law-making powers for the assembly and to review how Wales is funded and the criminal justice system.
Meanwhile, Tories say they have an "historic opportunity" in opposition.
Labour First Minister Rhodri Morgan has said the One Wales document negotiated with Plaid Cymru offered "stable and progressive government for the people of Wales".
Plaid AMs have supported plans to share power with Mr Morgan's administration, rather than ousting Labour from office by forming an alternative alliance with the Conservatives and Lib Dems.
The deal has been backed by both Labour AMs and the party's Welsh executive.
But after a private meeting of the Welsh Labour group, Caerphilly MP Wayne David said there had been "forthright" opposition, with most backbenchers voicing concerns.
Another source told the BBC that former party leader Neil Kinnock had been "absolutely devastating" in attacking the power-sharing document, One Wales.
The backbenchers are now likely to voice their criticisms at Labour's special conference next week.
The MPs were briefed by Secretary of State Peter Hain on the contents of the deal.
Lord Kinnock said he did not want to comment on the meeting.
Labour was left with 26 seats after the 3 May election, five short of an absolute majority in the 60-member assembly.
HOW THE ASSEMBLY STANDS
Labour - 26 seats
Plaid Cymru - 15
Conservatives - 12
Liberal Democrats - 6
Independent - 1
The combined Labour-Plaid administration would have 41 of the seats.
'Stale old policies'
Meanwhile the Conservatives, with 12 seats, are preparing to become the official opposition to the Labour-Plaid administration.
The Tories' strategy will be to try to establish themselves as the only real alternative to Labour.
Conservative assembly leader Nick Bourne will tell party members in Powys on Friday night that there is a "moral duty" to "rise to the challenge" of the new role.
At the meeting, in Brecon, he will argue that the chance to form a "progressive, consensus government" of the opposition has been "snatched away" and question whether Plaid could "exercise any real degree of control in the new government".
Mr Bourne will say: "Now, more than ever, we have a historic opportunity to rise to the challenge to protect our people and our country.
"It is clear that voters want an alternative to Labour, not merely a diluted version of the same (with Plaid).
"I am sure that voters will watch this new coalition closely to see whether Plaid can give Wales a radical alternative government, or whether we will just see more of the same tired, stale old policies".