Foreign minister Julie Bishop has been knocked out in the first three-way vote, sources told SBS News, clearing the way for a one-on-one contest between Peter Dutton and Scott Morrison.
Former Home Affairs minister Peter Dutton was the first to declare his candidacy, demanding a second ballot after losing to Mr Turnbull by seven votes on Tuesday.
The third choice is treasurer Scott Morrison, seen by some in the Coalition as a compromise between Ms Bishop's moderate views and Mr Dutton's conservatism.
Mr Turnbull has pledged to quit the parliament altogether if the spill motion succeeds today, potentially triggering a by-election in his Sydney seat and threatening the government's slim one-seat majority in the House of Representatives.
Legal advice bolsters Dutton's case, with caveats
The government's top lawyer, Solicitor-General Steven Donaghue, has declared Liberal leadership challenger Peter Dutton is likely eligible to sit in the parliament, bolstering the former Home Affairs minister's legal argument ahead of a highly anticipated leadership challenge.
The legal advice was provided to the government on Friday morning and forwarded to Mr Dutton.
"In my opinion the better view is that Mr Dutton is not incapable of sitting as a member of the House of Representatives by reasons of s44(v) of the Constitution," Mr Donaghue wrote.
But there were some caveats. The solicitor-general said it was "impossible to state the position with certainty" without more detailed facts about Mr Dutton's financial stake in two childcare centres.
Mr Dutton's case was unlike previous tests of Section 44, he said, and the seven justices of the High Court were clearly divided over the section's interpretation.
"There is a possibility, consistently with the approach that the High Court recently took in the context of s 44(i) of the Constitution [dual citizenship], that the Court might endeavour to create a clearer line in the interests of certainty, which might involve a broader reading," Mr Donaghue wrote.
Defiant Turnbull says right-wing staged 'insurgency'
In an extraordinary press conference on Thursday, the prime minister criticised rebel MPs within his party for staging an “internal insurgency”.
"I have never given in to bullies, but you can imagine the pressure it's put people under," he told reporters.
“They’re hard to stop,” he said.
“What we have witnessed at the moment is a very deliberate effort to pull the Liberal Party further to the right.”
Government in chaos as ministers resign
The crisis was amplified when key ministers Mathias Cormann, Michaelia Cash and Mitch Fifield deserted Mr Turnbull in a move that all but spelled the end of his prime ministership.
It came on Thursday as the Turnbull government successfully adjourned Parliament, cancelling Question Time in an extremely rare step that led to Labor accusations that the Coalition had lost the ability to govern.
Senator Cormann said it was with "great sadness and a heavy heart" that the three ministers advised the prime minister that he no longer had the support of the majority of the party room and offered to resign.
The three ministers repeated their concerns on Thursday morning after learning that five Cabinet ministers, who supported Mr Turnbull in Tuesday's ballot, had switched their allegiance.
"We advised the prime minister of our judgement, that he no longer enjoyed majority support in the party room, and that we believe we needed to - that he needed to facilitate an orderly transition, and that we should a party room meeting," Senator Cormann told reporters.
The influential finance minister said he now believed Mr Dutton was the best person to lead the party to the next election.
The three former allies of the Prime Minister join a growing list of ministers that have offered their resignation from the frontbench.
Mr Turnbull had tried to persuade ministers to continue in their roles even if they had backed his rival in Tuesday's ballot, but several are now determined to quit.
Health Minister Greg Hunt, who is likely to run as Peter Dutton's deputy in a second leadership ballot, is among the latest to go.
Trade Minister Steve Ciobo, Human Services Minister Michael Keenan and Citizenship Minister Alan Tudge have all been to see the prime minister to tell him they cannot to serve under him.
Assistant Minister James McGrath, a former Turnbull loyalist, quit for the second time on Wednesday and insisted the Prime Minister accept his resignation.
"Like Peter Dutton has said, we must do everything in our power to stop Bill Shorten ever becoming Prime Minister," Senator McGrath posted on Facebook.
Michael Sukkar and Zed Seselja have also insisted the Prime Minister accept their resignations after declaring their support for his challenger Peter Dutton.
Nationals MP threatens to quit a Dutton government
Meanwhile, a Nationals MP has pledged to quit the government and sit on the crossbench if there is another leadership spill, in a move that could threaten the Coalition's one-seat majority in the House of Representatives.
"The constant rotation of prime ministers by both the Labor Party and the Liberal party, I cannot condone," the Kevin Hogan, the MP for Page said.
He is one of several MPs to express frustration at the latest leadership crisis to dominate Canberra.
Since losing Tuesday's leadership ballot by 13 votes, Mr Dutton has been working the phones and embarking on a charm offensive in the media, trying to secure the extra seven votes he needs to roll Mr Turnbull.
He went on Melbourne radio in a bid to let voters know who he is, revealing his idea for a royal commission into electricity and fuel prices.
But his plan to exclude household electricity bills from the GST copped a belting.
"That would be a budget blower, an absolute budget blower," Treasurer Scott Morrison told reporters in Canberra.
He said it would deprive the states and territories of $7.5 billion over four years.
Shadow Treasurer Chris Bowen said Mr Dutton's "crazy" proposal would mean cuts to health and education.
Mr Dutton has also pledged to cut Australia's immigration intake.
“I think we do have to cut the numbers (of migration) back, I haven’t got a number to give to you today ,” Mr Dutton said.
“I think immigration is incredibly important to our country I think it needs to be operating in our country’s best interests”.
What comes next in the battle for the prime ministership?
* A leadership spill will be held during today's party room meeting at noon today if the prime minister receives a letter with 43 Liberal MP signatures asking him to do so.
* Before that Mr Turnbull expects to receive legal advice at 8.30am over leadership rival Peter Dutton's eligibility to sit in parliament.
* If it fails to clear Mr Dutton over a possible breach of section 44 of the constitution, the meeting will not go ahead and it is likely he would be referred to the High Court by the parliament when it next sits.
* In that case, Turnbull would remain leader and prime minister.
* If Dutton is not found to potentially be in breach of the constitution, it would clear the way for the Liberal partyroom meeting.
* Treasurer Scott Morrison and deputy Liberal leader Julie Bishop are canvassing support to run against Peter Dutton.
* Under party rules, the person with the fewest votes in a three-way race is knocked out, then it becomes a two-candidate vote.
* The winner of the Liberal partyroom ballot will become parliamentary party leader.
* Malcolm Turnbull would then tender his resignation to the governor-general and resign from parliament.
* The new Liberal leader would be recommended to the governor-general to become Australia's 30th prime minister.
* In parliament, which does not sit again until September 10, Labor could move a "no confidence" motion to test the government's majority. The Liberal-National coalition has a one-seat majority but some Nationals MPs have pledged to sit on the crossbench if the prime minister changes.
* The government already has budget "supply" until July 2019.
* The governor-general would issue the instrument of appointment and oath of office to the new prime minister, and most likely swear in a handful of key cabinet ministers.
* If Turnbull resigns immediately a by-election will be needed in the NSW seat of Wentworth.
* A half-Senate and full lower house election is due by May 18, 2019.
Malcolm Turnbull and China's President Xi Jinping. Chinese officials are bemused by Australia's string of leadership changes since 2007.