David Cameron has defended the use of "very robust" tactics in this month's election campaign, after angry Liberal Democrats accused Tories of trying to exploit the death of a teacher for political gain in a Cabinet row over knife crime.

The split between the coalition parties was exposed by a report in the Daily Mail, which revealed that Deputy Prime Minister Nick Clegg is blocking a proposal from Conservative Justice Secretary Chris Grayling for automatic prison sentences for offenders caught twice with a knife.

Letters obtained by the paper showed that Lib Dem Treasury minister Danny Alexander was opposing the policy on cost grounds, while the party's schools minister, David Laws, warned that mandatory minimum sentences were "too blunt an instrument".

An anonymous source said that Mr Clegg refused to allow discussion of the idea at a meeting this week of the Cabinet home affairs committee, which he chairs.

The report came as campaigning got under way for local and European elections on May 22, amid an atmosphere of heightened concern over knife crime following the fatal stabbing of teacher Ann Maguire in a Leeds classroom.

A senior Liberal Democrat source said: "Those people who have leaked this correspondence out of context and those seeking to exploit it on a party basis in such a tragic week will have to examine their own conscience."

The source described the policy as a "tough-sounding gimmick" that could turn youngsters into hardened criminals and actually increase crime.

" Whilst minimum sentencing might sound attractive in media headlines, there is a serious risk it could undermine the role of the judges who are best placed to decide on sentencing by virtue of their role," said the source.

"Sending youngsters automatically to jail regardless of the circumstances also has the potential to turn them into hardened criminals and can lead to more, not less, crime.

"The Liberal Democrats are interested in what actually works to tackle knife crime, rather than tough-sounding gimmicks."

Downing Street firmly rejected any suggestions that Mr Cameron's team were guilty of exploiting the Leeds killing.

The Prime Minister's official spokesman said: "I would not accept that."

Questioned about the row as he launched the Tory election campaign in Staffordshire, Mr Cameron insisted that coalition Government should not prevent a "robust" battle for votes between the parties.

"We are in coalition government and we have shown in coalition government that we can work together to deliver the plan that Britain needs," he said. "But when it comes to local elections and European elections, you can have a very robust fight."

He added: "On this particular issue, we have toughened the rules on knife crime. We should continue to look at that and I'm sure we'll reach a good outcome on that issue."

Mr Grayling wants mandatory jail sentences to be introduced through an amendment to the Criminal Justice and Courts Bill, which returns to the Commons on May 12.

Mr Clegg refused to allow the proposal to be debated a this week's meeting of the Cabinet committee on home affairs.

A senior Lib Dem source said the item had not been on the agenda and could be discussed in future if it was.

"Discussions about important areas of policy can't just be mused on ad hoc - they need to be properly debated," the source said.

The party claimed the support of Tory Cabinet minister Ken Clarke - Mr Grayling's predecessor as Justice Secretary - for its opposition to the move.

In a letter to Mr Clegg ahead of this week's committee meeting, Mr Alexander said he had "serious concerns" about Mr Grayling's plan, which he said would " impose a substantial new pressure on the prison population as soon as it is implemented".

"The Ministry of Justice's budget is already under significant strain and this proposal would certainly not be affordable before 2016/17," said the Chief Secretary to the Treasury Mr Alexander.

Meanwhile, Mr Laws told Mr Clegg he was "unwilling to clear this proposal", writing: "The principle of judicial discretion is the cornerstone of our sentencing framework and mandatory minimum sentences restrict such discretion and can lead to unintended or unwelcome outcomes.

Conservative backbencher Nick de Bois has tabled an amendment to the Bill along similar lines to Mr Grayling's proposals, which has already won the backing of 24 Tory MPs.

Mr de Bois told the BBC Radio 4 Today programme: " There are two proposals being put forward. One is for adults - 18 and over - which is a mandatory six-month minimum sentence. If you are younger than 18, the law would allow us to impose mandatory detention orders for four months, which is two months served and two months outside."

The Enfield North MP added: "I'm afraid the discretion of the judges, in my opinion, and I believe of many other people, is rather inadequate when we actually see sentencing of something like 4,500 people getting community sentences, 3,200 people just getting a caution and a fine. That's not satisfactory."

Education Secretary Michael Gove told Sky News: "It is really important that we send a very clear signal that knife crime is going to be dealt with effectively.

"It is absolutely important that we use Parliament to communicate to the public - and to anyone tempted to carry a knife in public - that the sentence for behaving in this way will be clear and firm and tough."