But a group of rebel Liberal senators yesterday defied their leader and crossed the Senate floor on four motions relating to the scheme.

In the biggest sign of dissent, 12 Liberal senators crossed the floor to vote for a motion proposed by NSW National John Williams to delay a vote on the ETS until after next month's Copenhagen climate conference.

The motion lost 52 votes to 18, with the backing of at least 10 Liberal senators - enough to pass the emissions trading scheme in a likely vote today.

Notably, Liberal Senate Leader Nick Minchin and deputy Senate leader Eric Abetz - both climate sceptics but powerful members of the Coalition - were absent from chamber during votes where Liberal senators crossed the floor.

Three shadow parliamentary secretaries - WA senator Mathias Cormann, Victorian senator Mitch Fifield and Queensland senator Brett Mason - resigned from the Liberal front bench yesterday, ahead of the vote to defer the legislation to after Copenhagen, so they could vote against their party. "Our decision is made with regret but also in clear conscience," the senators said in a statement.

Speaking in the chamber, Senator Cormann - who was the shadow parliamentary secretary for health administration - said he had called Mr Turnbull yesterday morning to tell him he could not vote for the scheme before an international agreement on climate change was reached.

The shadow parliamentary secretary for Northern Australia, Queensland senator Ian Macdonald, also crossed the floor to defer the legislation.

He is now likely to lose his position under Liberal Party convention to strip frontbenchers of their title for crossing the floor.