Labour leader Ed Miliband is "sympathetic" to moves to introduce tougher knife crime sentences that have caused deep divisions in the coalition.
Conservatives want to introduce an automatic jail term for offenders caught twice with a knife, but Deputy Prime Minister Nick Clegg is opposed to the plans.
Britain's top policeman Sir Bernard Hogan-Howe has backed the proposals, saying similar measures for gun offences led to a drop in the number of young people being picked up with the weapons.
Lord Chief Justice Lord Thomas, the most senior judge in England and Wales, has also called for an urgent review of sentencing for youths carrying knives, saying it was a "major problem" among 12 to 14-year-olds.
Conservative MPs are now trying to introduce the reform by amending a bill making its way through parliament and Labour support could see it pass in the face of Liberal Democrat attempts to block it.
Mr Miliband, however, insisted any changes would have to be " done in the right way" and the party has not said what it will do in Monday's vote.
The Labour leader told the BBC: "I don't agree with Nick Clegg. I think we do need tougher penalties for knife crime. Labour's been consistently arguing that this is an issue that needs to be looked at.
"We definitely do want to use the legislation to strengthen the law and we are definitely sympathetic to the issue of mandatory sentences after a second offence, after conviction for a second offence.
"We need to make sure it's done in the right way, but that's where we stand, because I think the public want to know that we are going to send a strong message to people who carry knives repeatedly that it is not acceptable."
Sir Bernard, Metropolitan Police Commissioner, said it was " vital" to send a clear message to young people.
He told LBC: "They do hear simple messages. They have heard it about guns, I think they need to hear it about knives."
Conservative mayor of London Boris Johnson said: "Nick Clegg is wrong. I do believe it's vital that we work to educate and rehabilitate those caught up in the culture of knife-carrying but equally it is imperative that the police and the courts are given every support to tackle the scourge of knife crime."
David Cameron's official spokesman appeared to signal that the Prime Minister had some sympathy for an amendment to the Criminal Justice and Courts Bill tabled by backbencher Nick de Bois and backed by 39 Tory MPs, which would put the proposal for mandatory jail sentences into effect.
Asked whether the PM would encourage Tories to support the Enfield North MP's amendment, the spokesman told a regular Westminster media briefing: "With regard to that amendment, that is something the Prime Minister is going to give very detailed consideration to."
Liberal Democrats have accused the Tories of seeking to exploit the death of teacher Ann Maguire in a Leeds classroom for political gain.
Mr Clegg insisted he is "not soft on knife crime at all" but warned that mandatory sentences could lead to innocent peopled being jailed.
On his regular LBC Call Clegg show, he added: "Yes, let's be really tough on knife crime but let's be smart about how we are tough on knife crime. Let's not just simply catch headlines in a way which doesn't necessarily deal with the problem."