At the weekend, the Act Party's board heard a presentation from former leader Rodney Hide - who opposes the bill - and passed a resolution in support of freedom of speech and the right to silence.
Act president Chris Simmons said MPs who voted for any bill that potentially removed the right to silence - even if responsibility was delegated away from Parliament - would be voting against the clear position of the party.
"We're opposed to the removal of that right to silence, but the party doesn't direct MPs how to vote," Mr Simmons said. "It puts us in a very tricky position."
Party leader Don Brash said the board's resolution was not a warning to its MPs.
"Clearly all three of the [MPs] are not contesting the election this year. We certainly can't say, 'If you don't do this, we won't have you on our list'. I'm sure they won't lightly make a decision which contradicts the board."
The bill would require the defence to disclose to the prosecution before a trial all disputed aspects of the case. Failure to do so would enable a judge or jury to infer a greater likelihood of guilt.
At present, a defendant can stay silent, leaving the case to the prosecution to prove beyond reasonable doubt.
Auckland University emeritus law professor Jim Evans said delegating responsibility to the Rules Committee was "wrong".
"Constitutionally, it's totally inappropriate. To send off a matter of this nature - a matter of fundamental principle and over which Parliament itself will not produce a majority - to some delegated authority is wrong."
Mr Key seemed to pour cold water on the Rules Committee proposal.
"I wouldn't jump to the conclusion that that would happen yet," he said.
The PM said if the numbers could not be found, the bill would be held over for the next parliamentary term.
BILL'S PROGRESS
Last week The Government appears to secure the numbers for the Criminal Procedure Bill, which critics say erodes the right to silence. The bill appears at the top of this week's order paper.
Weekend Act Party board passes a resolution supporting freedom of speech and the right to silence.
YesterdayThe numbers situation is at a knife edge again and the PM says he does not expect the bill to progress this week.