The Conservative government is expressing outrage after Liberal senators amended a piece of "tough on crime" legislation that had already passed the House of Commons -- and that Liberal Leader Michael Ignatieff had supported.

Part of the original bill would eliminate the current two-for-one sentencing credit given to convicts who spend time in jail before their conviction.

The Senate justice committee voted to change that to time-and-a-half served, meaning convicts would get 1.5 days' credit for every day served pre-sentencing.

The amendment would also give judges discretion in awarding pre-sentencing credits, and would require an explanation for their decision.

The amendments have yet to be voted on by the full Senate, which is controlled by the Liberals, despite Prime Minister Stephen Harper's recent appointments to the Upper Chamber.

Justice Minister Rob Nicholson said the amendments "basically gutted the bill."

"This what I've been worried about," he added.

The Senate's mandate is to review -- and amend if it chooses -- legislation before it becomes law. The House of Commons then has to approve any amendments.

Nicholson has called on Ignatieff to intervene and force the Liberal senators to drop the amendments.

"He should impose some discipline down there," he said.

Ignatieff supports the tough-on-crime bill. The move by Liberal senators only enhances the perception that the Liberal party is not fully behind its leader.

"I continue to believe that the bill should be supported unamended and that was Mr.Ignatieff's position every time it was voted in the House of Commons," Liberal MP Dominic LeBlanc said Wednesday.

Furthering the intrigue is that some of the senators supporting the amendments supported Liberal MP Bob Rae in his failed leadership run. Rae has refused to criticize the senators' decision, leaving the impression of a divide between him and his former rival Ignatieff.

Liberal Senate spokesman Marc Roy said the justice committee did its job based on the testimony.

"They are there to be independent," Roy said.

"If these (amendments) don't happen every once in a while then we really don't have an independent chamber or review process."