Communications shadow minister Nick Minchin has resigned his portfolio in protest over the Coalition's stance on emissions trading.
The move reversed earlier assurances made by his office that any such move was hypothetical and purely speculative.
In a statement, Minchin said he and Shadow Minister for Families, Housing, Community Services and Indigenous Affairs, Tony Abbott, had proposed the leader of the opposition, Malcolm Turnbull, "reconsider the Coalition’s policy position in relation to the carbon pollution reduction scheme legislation".
Minchin proposed the legislation be referred to a Senate committee for inquiry and further consideration of the Bill be given only after the United Nations climate change conference in Copenhagen next month.
"Mr Turnbull declined this proposition so I advised that I would have no alternative but to resign from the Shadow Cabinet as I was not able to support the CPRS legislation," Minchin said.
The resignation was effective as of the end of parliament this year.
However, he would remain Senate Leader until at least the resumption of the Senate in February next year. Minchin advised he would then step down, enabling senators to elect a new leader.
Minchin said that Turnbull had accepted his resignation.