Fianna Fáil leader Micheál Martin says he has 'no mandate' to speak to Sinn Féin about government formation.
Deputy Martin - who could become Taoiseach this weekend if Fine Gael, Fianna Fáil and the Greens approve the programme for government - also said there's no Plan B if the plan is rejected.
Fine Gael leader Leo Varadkar earlier said his party has no back-up plan if the current programme for government isn't approved.
However, he suggested that Fianna Fáil and Sinn Féin could be in government together "within weeks" if his party rejected the current deal.
Mary Lou McDonald, meanwhile, yesterday said Sinn Féin will again attempt to form a government if Fine Gael, Fianna Fáil and the Greens fail to do so.
Speaking on Newstalk Breakfast, Deputy Martin said he's been given clear instructions by his own party.
He said: "I have no mandate to speak to Sinn Féin. Our parliamentary party gave me a clear mandate to engage with Fine Gael and other like-minded parties in terms of policies.
"The detail that we've engaged in with the Green Party and Fine Gael has been quite exhaustive - it's not as simple as just turning a switch and saying we'll go somewhere else.
"We're focused on this - it's not up to Fianna Fáil alone in the aftermath of this if it doesn't work out, [but] I'm hopeful it will."
'Good prospect' of deal passing
The Fianna Fáil leader said he believes there's a 'good prospect' all three parties will vote for the deal - but admitted it will be challenging.
He said: "I'm not counting any chickens before they hatch.
"We obviously have three votes - one within the Green Party membership, one within Fine Gael and one within Fianna Fáil. I'm focusing on the Fianna Fáil one, in terms of asking our members to support this programme for government.
"The Green Party with a two-thirds threshold is more challenging."
Deputy Martin said the country is in "deep trouble" in terms of the fallout from the COVID-19 pandemic, and therefore a government needs to be formed.
If the deal isn't passed, Deputy Martin observed: "I think we're in a difficult situation if that happens, and I think we will be into a political crisis.
"I certainly don't have a crystal ball to say what will happen... alternatives will not be easy at all."
Responding to Leo Varadkar's suggestion that he'd like Fine Gael to retain the housing portfolio, the Fianna Fáil leader said the three party leaders haven't yet decided who will get which ministry.
He said: "We're very interested in housing as well, and I think the three leaders have to meet to work through that issue in terms of departments... nothing has been decided in relation to that.
"Suffice to say we would have our priorities as a party in terms of the various portfolios."