Mrs Mirabella said hundreds of emails and calls from constituents concerned about Opposition amendments to the scheme helped force her hand.

The move would give her the freedom to vote against the scheme.

“There is one thing more important than your position in Parliament and that is knowing the time you spent sitting on the green leather was for the betterment of the nation and for the betterment of the people you represent,” she said.

“It is an issue that has caused great concern for many Australians and that is reflected in their MPs’ thinking.

“What others will do as a result I don’t know.”

When she does so, she will cross the floor for the first time in her parliamentary career, but she was required to first relinquish her role as Opposition spokeswoman for early childhood education, child care, women and youth.

Mrs Mirabella was one of 12 frontbenchers to last night offer their resignations to Opposition Leader Malcolm Turnbull, the other major ones being Tony Abbott and Liberal Senate leader Nick Minchin.

She said she had spoken with Mr Turnbull just after 5.15pm yesterday, when he had received her letter.

“It was all very civil but my comment is very clear,” she said.

“I can’t do anything else but vote against Labor’s emissions trading scheme.”

Mrs Mirabella had been guarded in her comments earlier this week following Tuesday’s party room meeting, saying as a member of the shadow ministry, she endorsed the party room decision to back the trading scheme deal between the Coalition and Labor.

But yesterday, she said she believed there had been a clear majority in the party room in favour of voting against the legislation.

“I accept the leader has the role of interpreting the will of the party room. That said, I cannot personally support his decision to vote for Labor’s emissions trading scheme,” she said.