The Nationals claimed Opposition Leader Sussan Ley "mismanaged" a spat over hate speech laws leading to resignations en masse from Ms Ley's frontbench.
Mr Littleproud joined 10 other frontbenchers from his party in quitting Ms Ley's shadow cabinet on Wednesday, after she accepted the resignations of three senators earlier that day.
But on Thursday he declared his party "cannot be part of a shadow ministry under Sussan Ley".
"She has also forced the coalition into an untenable position that can no longer continue," he told reporters in Brisbane.
"The National Party is our own sovereign party, and Sussan Ley cannot force us to vote one way or another."
In a statement, Ms Ley said the focus should be on Jewish Australians as Australia mourns the victims of the Bondi terrorist attack.
"This is a national day of mourning and my responsibility as leader of the opposition and leader of the Liberal Party is to Australians in mourning," she said.
It's understood Ms Ley spoke with Mr Littleproud ahead of his press conference to urge a 24-hour pause in respect of the day.
It is the second time since the 2025 federal election the coalition has been on the brink, after a brief divorce between the political parties last May.
Nationals senators Bridget McKenzie, Ross Cadell and Susan McDonald resigned from their portfolios on Wednesday after breaching cabinet solidarity, a principle that requires shadow ministers to all vote the same way.
Senator McKenzie said it had been made clear to Ms Ley what would happen if she accepted the three initial resignations.
"She made that decision knowing what the consequences would be, and you've seen that play out," she told reporters in Canberra.
"We've all made it very, very clear that we are coalitionists.
"This is a decision for Sussan Ley, and that is obviously something that she's going to have to be considering."
The Nationals who have offered to quit would leave their portfolios but remain in the party as backbenchers.
The extraordinary rift between the two coalition partners was triggered by a disagreement over the government's anti-hate laws drawn up in the wake of the Bondi Beach terror attack.
The coalition's shadow cabinet agreed to back the bill during a meeting on Sunday, but two days later the Nationals announced they would oppose it because of concerns about its impact on free speech.
Mr Littleproud wrote to Ms Ley the same day, warning all remaining shadow ministers from his party would quit if she accepted the trio's resignations.
"As it was a partyroom decision, if these resignations are accepted, the entire NationaI party ministry will resign to take collective responsibility," he wrote.
At an emergency meeting on Wednesday night, the remaining eight Nationals frontbenchers decided to quit their portfolios in protest.
The temporary split in May 2025 was triggered by a number of demands being made by the rural party, including an ongoing commitment to nuclear power and a pledge to introduce forced divestiture powers for the supermarket sector.
The two parties reunited after a week, but insiders believe the next separation has the potential to last much longer.
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