A former top prosecutor says he personally warned Boris Johnson about the risk posed by freeing terrorists who had not been deradicalised, but was told there was “no money”.

The hugely damaging claim came as the solicitor for the London Bridge attacker revealed he had asked for help to turn away from terrorism while in prison, but was not given any.

As anger grew over the early release of Usman Khan, Jeremy Corbyn branded the attack he was able to carry out “a complete disaster”, saying: “There has got to be a very full investigation.”

Visiting London Bridge, the prime minister attempted to deflect blame, saying: “I’ve argued that when people are sentenced to a certain number of years in prison they should serve every year of that sentence.”

But the focus switched to efforts made to deradicalise prisoners when Nazir Afzal, the former chief prosecutor for North West England, intervened by revealing his private conversation with Mr Johnson.

He said he had raised the problem of terrorists being released “whilst ostensibly rehabilitated but still radicalised” in many government meetings, before raising it with Mr Johnson in June 2016.

“He asked me what keeps me awake at night and I told him it was this issue,” Mr Afzal said.

“When he wanted to know what to do about it, I told him it was more resources for one-to-one deradicalisation.

“Back then, he hadn’t found the ‘money tree’ so he frustratingly said there was no money.”

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The atrocity committed by Khan – who killed two people, before being shot dead by police on Friday – has become a major general election controversy, with questions also asked about the apparent failure to monitor him.

Meanwhile, the Conservative campaign lead has been halved in just one week, putting the UK in “hung parliament territory”, an exclusive poll for The Independent showed.

The criticism of the background to the atrocity grew as University of Cambridge graduate Jack Merritt was named as one of the people stabbed to death, at a conference on prisoner rehabilitation.

A woman who died in the attack has not yet been named, while three others were injured and remain in hospital, two in a stable condition and one with less serious injuries.

Khan was originally given an indeterminate sentence in 2012, for his part in an al-Qaeda-inspired plot to bomb the London Stock Exchange, meaning he would remain locked up for as long as it was felt necessary to protect the public.

But this sentence was quashed the following year and he was given a 16-year jail term, allowing him to be automatically released after serving eight years.

Further questions were raised when the Parole Board said it had no involvement when Khan left prison last December, saying he “appears to have been released automatically on licence”.

Chris Phillips, a former head of the UK National Counter Terrorism Security Office, warned the criminal justice system was “playing Russian roulette” with the lives of the public.

Pointing out that the original trial judge “wanted this man in prison for a very very long time”, he described Khan’s release as “quite incredible”.

“What we have got to ask now is why is the criminal justice system allowing people like him to be back on the streets?” Mr Phillips said on Sky News.

Meanwhile, Khan’s solicitor said he had come to realise that violent extremism was wrong after being jailed. “He requested intervention by a deradicaliser when he was in prison,” Vajahat Sharif told The Guardian.

“The only option was the probation service and they cannot deal with these offenders. He asked me on the phone to get assistance from a specific deradicaliser.”

Mr Sharif suggested extremists may have targeted Khan to be “regroomed” after his release from jail last December, with devastating consequences.

“In prison he begin to realise his Islamic thinking was not correct; he accepted that. He criticised the al-Qaeda ideology and violent extremism,” he said.

On the campaign trail in Leeds, the Labour leader said: “Clearly there has been a complete disaster in that lives have been lost because of his [the terrorist’s] behaviour.

“I think there is also a question about what the probation office were doing – were they involved at all – and whether the Parole Board should have been involved in deciding whether or not he should have been allowed to be released from prison in the first place, and also what happened in prison?

“That somebody who clearly was a danger to society ... was he given a deradicalisation programme or not?