Boris Johnson has suggested the London Bridge attacker may have been “too tough to crack”, as leaders of the main parties were sharply criticised for “politicising” the terror incident in the run up to the General Election.

The Prime Minister’s latest comments about attempts to rehabilitate Usman Khan appeared to be a rowing back from what Mr Johnson has previously said about the convicted terrorist being back on the streets because of laws introduced by a “leftie government”.

Following a series of heated changes over the weekend involving Mr Johnson and Jeremy Corbyn, Dame Louise Casey – a former government adviser on community cohesion – said both sides were oversimplifying the issue.

And former Tory justice minister Phillip Lee – who has since defected to the Lib Dems – launched an attack on Mr Johnson, accusing him of “lying and misleading” in the wake of the tragedy.

“The desperate, sort of, politicisation of this by Boris Johnson – not a man who is known for details – wading into something which is actually quite complex is not appropriate,” he told the PA news agency.

On Monday afternoon, Mr Johnson took questions from the media and it was pointed out to him that the family of victim Jack Merritt have asked for his death not to be used to justify introducing “even more draconian sentences” on offenders.

Incident on London Bridge
London Bridge victims Jack Merritt and Saskia Jones (Met Police/PA)

Asked what happens to those people who can never be rehabilitated, Mr Johnson replied: “I’m just saying that in his case (Khan), I think that probably is true that people can’t be rehabilitated and I think it varies very widely.

“There are unquestionably some cases which are just too tough to crack and alas he appears to have been one of them, and I’m afraid it was probably clear from the outset that that would be so.”

Asked what should be done with such offenders, Mr Johnson said: “I think you have to do what you can and a great deal of effort was gone in to to try to change him and try to change his ways, but in the end better, I’m afraid, for the protection of the public, better for the protection of society and of us all to keep him in (prison) rather than run the risk of letting him out.

“That’s what we’re proposing.”

Jo Cox’s widower Brendan Cox tweeted: “When there’s a terror attack it’s right politicians say what they would do to stop them in future and how they would look after victims. What we don’t need is cheap point scoring or blame shifting.”

The Tories have sought to blame legislation passed under the last Labour government for the early release of Khan, which meant he was let out of prison halfway through his sentence.

Labour in turn accused the Conservatives of starving the prison and probation services of resources, warning that it was impossible to “keep people safe on the cheap”.

Mr Corbyn has also linked the radicalisation of Khan – who was convicted of terrorist offences in 2012 – to the Iraq war.

However, Dame Louise said the issues were more complex than either side was prepared to allow.

“What happened over the weekend, the politicisation of this – Jeremy Corbyn saying this is all about austerity and today saying it’s all about the Iraq War, and Boris Johnson saying this is about being tough on crime and longer prison sentences – the truth is these things are much more difficult,” she told BBC Radio 4’s The World at One.

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Dame Louise Casey has criticised the party leaders for politicising the London Bridge terror attack (Ian Nicholson/PA) 

“It has got very political, very quickly about what is happening in prisons.

“Either you have to say this is all about cuts to the Prison Service and it is all about re-offending or you are saying ‘Actually there is no hope, these people are going to be dreadful when they come out, we should never the let them out of prison’.

“I think the answer lies somewhere in the middle. Actually I don’t think we are resourced in prisons sufficiently to deal with this.

“At the same time I actually happen to agree that it should never have been an automatic that somebody was released halfway through their sentence for a terrorist offence.”

Mr Johnson and Mr Corbyn both attended a vigil at Guildhall Yard in London on Monday to honour Cambridge graduates Mr Merritt and Saskia Jones, who died when Khan launched his attack on a prisoner rehabilitation event they were supporting.

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Justice Secretary Robert Buckland has said it was important to get the ‘tone’ of the debate right (Aaron Chown/PA)

Mr Corbyn, campaigning in his Islington North constituency, said convicted terrorists should have to serve a “significant proportion” of their jail sentences, and should only be released when they have been properly rehabilitated and present no threat to the public.

“I do think that continuing with the process allows people to be released ahead of final completion of their sentence if they’ve been rehabilitated and they have been suitably assessed and they are very strictly monitored when they come out – I think that must be the correct way of doing things,” he said.