The Progressive Conservatives have already been using delay tactics to try to prevent the NDP government from passing a law that would raise the provincial sales tax July 1.
The legislature is scheduled to rise June 13, but Tory Leader Brian Pallister says July 1 is an important date.
He says if that date comes and the government still hasn't passed its law, the tax increase could be illegal.
Pallister says his caucus members are ready for an extended sitting because they believe in the fight.
The government has said the tax increase would be legal regardless of when the bill was passed.
The legislation would raise the PST to eight per cent from seven per cent and would eliminate part of the balanced budget law that requires a referendum on any sales tax increase.
The Tories and the Canadian Taxpayers Federation have talked about possibly taking the government to court.
Both groups said Monday they have not made a decision.