Ben Carroll confirmed on Monday he would nominate to lead the Victorian Labor Party and take over the premiership from Ms Allan.
In a post to social media, Mr Carroll said his colleagues had asked him to stand after informing him the premier had lost the support of the caucus.
"It is clear that the premier has lost the majority of our caucus," he wrote.
"As such, I have since visited the premier to inform her that I will be nominating for the leadership of the Labor Party."
A leadership challenge could take weeks to resolve and hamstring the state government as it vies for a historic fourth successive term in government.
But Mr Carroll said the stakes were too high not to step up, suggesting a loss at the November state election would pave the way for a three-way coalition between the Liberals, Nationals and One Nation.
Ms Allan refused to resign after a cross-faction delegation informed her she had lost majority support of the caucus, paving the way for a leadership spill on Tuesday.
She announced her intention to nominate should it go to an internal vote in a letter to the state secretary of the party on Monday morning.
Ms Allan requested the secretary provide support for a ballot of eligible party members and announced her intention to stand.
"I also wish to formally notify you of my intention to be a candidate for the leadership of the state parliamentary Labor Party, in the event that an election is required," she wrote.
The premier stared down previous leadership rumblings in June after poor polling pointed to a likely electoral defeat and her lowly popularity rating.
Mr Carroll publicly ruled out a challenge at the time, throwing his weight behind the premier.
Standing beside Ms Allan, the deputy premier denied he was coming for her job or that he would ask her to stand aside.
"I can look you in the eye and I can say that, premier," he told reporters at the time.
Attorney-General Sonya Kilkenny, a close ally of Ms Allan, shrugged off the leadership speculation as gossip on Sunday.
"She has my full support, and this idle gossip doesn't help Victorians," she said.
Federal Liberal senator James Paterson, from Victoria, said a change of leadership would not revive the party's chances at the November poll.
"It sounds like the factional bosses have finally caught up with the sentiment of Victorians, which is they want this premier gone," he told ABC Radio.
"But in my view, it's too little, too late."
He said Mr Carroll, as deputy leader, was tarred with the same brush as Ms Allan in the minds of voters and a change of government was needed.
Ms Allan has been under sustained pressure to call a royal commission into alleged corruption on major Big Build infrastructure projects across the state.
The premier has repeatedly ruled out doing so, arguing a more immediate solution was needed and that an inquiry would not lead to cultural change.
The latest Resolve Political Monitor earlier in July found 43 per cent of Victorians wanted Opposition Leader Jess Wilson as premier, against 24 per cent for Ms Allan.
The survey of 1000 Victorians put the primary vote of both major parties neck-and-neck at 27 per cent, compared with 22 per cent for One Nation.
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