Labour leader Andrew Little will try to call the Prime Minister's bluff by trying to introduce a bill to include Red Peak - but also change the order of the flag referendums' questions.
His move comes after Prime Minister John Key said he would consider including Red Peak if other parties in Parliament agreed - especially Labour.
Labour had criticised the flag change process from the outset, arguing it cost too much and that the first referendum should ask whether people wanted to change rather than waiting until the second.
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Mr Little responded to Mr Key's challenge by saying Labour would support it, but only if the change question was included in the first referendum.