Former Nationals leader Barnaby Joyce has quit the party, citing fractured relationships with its leadership.
The high-profile New England MP will not stand for his seat at the next election but does plan to see out the rest of the term.
"My relationship with the leadership of the Nationals in Canberra has unfortunately, like a sadness in some marriages, irreparably broken down," the former deputy prime minister wrote in a letter to branch members.
Reports the firebrand MP plans to defect to Pauline Hanson's One Nation were not confirmed in the statement but his options have been kept open.
"I am free to now consider all options as to what I do next," he said.
Joyce was demoted from the party's frontbench by current leader David Littleproud earlier this year.
He has been an outspoken critic of net zero emissions-reduction goals and been calling on the federal Coalition to abandon it.
His party's ongoing support of reducing Australia's emissions to net zero by 2050 was flagged as a sticking point.
"Our position in continuing to support net zero with the massive schism and hurt to my electorate, to small businesses, to the environment, to the poor, to the defence of Australia and creating hate between lifelong friends in my community makes continuing in the Nationals' Party Room in Canberra under this policy untenable."
Both the Liberals and the Nationals have announced reviews of their net-zero policies, which are still ongoing.
The move by Nationals leadership to instruct Joyce not to campaign outside his electorate during the last federal election was also criticised by the veteran MP.
Joyce's departure is the latest display of internal discontent within the federal Coalition's ranks after its worst election result in its 81-year history.
It follows Liberal leadership aspirant Andrew Hastie quitting the Opposition frontbench in early October after criticising his party's stance on net zero.
Conservative NT senator Jacinta Nampijinpa Price was dropped from the frontbench in September after refusing to apologise for controversial comments about Indian migrants.
'Friend, fellow warrior and courageous conservative'
National Party Senate leader Bridget McKenzie said today was "a sad day both for me and for our great party".
"I consider Barnaby a friend, a fellow warrior and a courageous conservative in the federal parliament as a member of The Nationals," she said in a social media post.
"We have fought many battles together and have had our own fierce policy debates.
"Throughout it all, I have found Barnaby a man of faith, of passions, and I regard him as a leader with a deep and abiding love and patriotism for our country."
Nationals Senate leader Bridget McKenzie said Barnaby Joyce leading the party in both houses and serving as deputy prime minister was "no small achievement". Source: Facebook / Senator Bridget McKenzie
"You know the Coalition is bad if not even Barnaby Joyce wants to be part of it," he said.
