Labour MPs have told First Minister Rhodri Morgan they back him in talks on a possible Plaid coalition, but they want to be consulted at every stage.
At the Westminster meeting, Caerphilly MP Wayne David said MPs understood Mr Morgan was in a difficult position but there was support for him in the talks.
Mr Morgan told MPs that Labour faces a choice between forming a coalition with Plaid Cymru or entering opposition.
Labour's Welsh executive will meet on Friday.
It will discuss a potential coalition and the calls for a special party conference.
Mr Morgan travelled to Westminster to meet the group of Labour MPs on Wednesday evening.
Mr David said that no vote on a possible coalition was held but the consensus was to support Mr Morgan in his discussions.
He added that proper consultation was needed with the party's rank-and-file members.
However, it is understood there was strong opposition to any offer of a referendum on giving the assembly full law-making powers.
Before the meeting, one senior MP said there must be no "rushing" into a deal and "no blank cheques".
"We want to know what he has put on offer, what discussions he has had on policy, we want the maximum consultation," said the source.
Welsh Secretary Peter Hain and Labour AMs want a special party conference to be held to consider any deal.
A Labour spokesperson said when the party's assembly group had met on Wednesday morning there had been "unanimous support" for holding a special party conference.
Mr Hain has previously ruled out the possibility of a Lab-Plaid coalition, saying he did not think the party's grassroots would accept it.
But initial proposals have been exchanged between the two parties ahead of detailed negotiations on a possible deal, which are expected to begin on Thursday afternoon.
Plaid AMs have given unanimous support for the talks with Labour, and are understood to have been guaranteed a referendum on full assembly powers.
A UK government source said: "There are some things that are unpalatable but there is the political reality of having 26 seats (five short of a majority).
One former minister warned there was no degree of enthusiasm for a Plaid coalition, but added he could tolerate it if it was endorsed by Welsh Labour as a whole.
The two largest parties in the Senedd may form a coalition
He added an alliance with Plaid was "a sea-change on a par with a Labour coalition with the Tories in London".
Plaid leader Ieuan Wyn Jones, who is also talking to Tories and Lib Dems, said parties must set aside their rivalries.
Mr Jones confirmed that Cabinet seats for his party would be part of any deal with Labour, and Plaid's 15 AMs backed continuing discussions with Labour at a meeting on Tuesday night.
Mr Jones told BBC Radio Wales that Labour had offered "fresh information that is interesting and we need to explore that now".
He said it was "difficult to go into detail" about what was on the table, but said his concern was a deal that was "in the best interests of the people of Wales".
However, he said a referendum on a full parliamentary system would be a "key demand" but added any deal would be about "a programme for government", not just on a single issue.
Mr Jones said it was about "securing a good deal for the people of Wales" and he was aiming to create a stable government.
The development is one of a series of twists and turns in Cardiff Bay since the 3 May election.
Mike German, leader of the six Liberal Democrats in the assembly, said Labour were "desperately keen on making sure they hang on by their fingertips to power".
Welsh Conservative leader Nick Bourne said: "Our position remains unchanged. We think Wales will be best served by a non-Labour alternative."