9News understands that Cook became the state’s 31st leader, and Health Minister Amber-Jade Sanderson dropped out of the race.
Sanderson was in first position to replace him this morning, beating Cook with the backing of their shared faction, the United Workers Union, by a vote of 17 to 11.
However, Cook managed to rally support in the afternoon with Transport Minister Rita Saffioti endorsing him for the job if she could be his deputy.
The Metal Workers Union also endorsed Cook, as well as Cabinet member Stephen Dawson.
Sanderson withdrew from the race following those decisions.
A Labor caucus meeting will be called next week when the new prime minister is officially decided, however Cook is the only candidate.
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