Brace yourselves, Australia. One of Turnbull's biggest allies just ditched him.

Malcolm Turnbull‘s days as prime minister are effectively over, after key cabinet allies abandoned him.

Mathias Cormann, Michaelia Cash and Mitch Fifield have resigned from cabinet and told the prime minister he no longer has majority support in the Liberal partyroom.

Former cabinet minister Peter Dutton has sought a fresh partyroom ballot, following his unsuccessful bid on Tuesday, but the prime minister has yet to allow the meeting to go ahead.

“I can’t ignore reality,” Senator Cormann told reporters in Canberra on Thursday, after meeting with Mr Turnbull.

“When I have five cabinet colleagues telling me they supported Peter Dutton on Tuesday … that is not something I can ignore.”

He said he had taken the action “with great sadness and a heavy heart”.

Senator Cormann, who on Wednesday publicly expressed his support for the prime minister at a media conference, said he had initially took it upon himself later on Wednesday to tell Mr Turnbull he no longer was supported by party colleagues.

He said Mr Dutton represented the best chance of winning the next election.

“We’ve made that judgment and it is now a matter for the party room,” he said.

The two other ministers expressed their support for Mr Dutton.

The former home affairs minister lost a challenge 48 votes to 35 on Tuesday but now wants to have another go at toppling his leader.

“Earlier this morning, I called the prime minister to advise him that it was my judgment that the majority of the party room no longer supported his leadership,” Mr Dutton told reporters at Parliament House on Thursday morning.

“As such, I asked him to convene a meeting of the Liberal Party at which I would challenge for the leadership.”

It is expected Mr Turnbull will resign, but his office has yet to comment.