The “destruction of the prison and probation service” under the Conservatives lay behind the threat, said Ian Acheson, who carried out a review for the government, adding: “You cannot get away from that.”
The cause was “privately acknowledged” by ministers, he said, urging them to admit publicly: “We went far too far, far too fast – we are now reaping what we sowed.”
Mr Johnson has blamed Labour sentencing policy and claimed only a Conservative general election victory can keep the UK safe, triggering accusations that he is trying to weaponise the tragedy for political gain.
But Mr Acheson said he was “depressed” by what was turning into an “arms race on sentencing”, arguing it ignored the crucial issue of what goes on in prisons and probation.
“What we need to be focusing on is the capability of the public protection services, from the first night of the terrorist offender’s time in custody to the last day of his supervision in the community,” he urged.
Having advised governments across the world, Mr Acheson said he was “shocked” by what he found in British prisons when he carried out his review for Michael Gove, in 2015.
He attacked “a lethal combination of arrogance, ineptitude and defensiveness in Whitehall”, with “fearful” staff struggling because “training simply didn’t exist”.
“At the heart of this is the destruction of the prison and probation service through crazy, failed, ideological austerity cuts,” he told BBC Radio 4’s The World This Weekend programme.
Mr Acheson, who described himself as “a Conservative”, said he had made 69 recommendations, but they were “conflated” into 11 in an official response to his report.
Earlier, Mr Johnson denied steep cuts to the criminal justice system had been “a mistake”, replying: “No.”
“It was nothing to do with parole, nothing to do with the probation service,” he insisted, on the BBC’s Andrew Marr Show, referring to Khan’s attack.
The prime minister’s focus purely on sentencing rules inherited from Labour – which he wrongly claimed left judges with “no option” but to grant Khan automatic early release – has drawn increasing criticism.
Ed Davey, the Liberal Democrat deputy leader, urged Mr Johnson stop “trying to make political capital out of a tragedy” and to apologise for “misleading people” over the law regarding early release.
On Saturday, Nazir Afzal, the former chief prosecutor for North West England, revealed he had been rebuffed when he warned Mr Johnson about failing to work properly with dangerous prisoners.
He had raised the problem of terrorists being released “whilst ostensibly rehabilitated but still radicalised” in many government meetings, before warning Mr Johnson in June 2016.
“Back then, he hadn’t found the ‘money tree’ so he frustratingly said there was no money,” Mr Afzal said.
Mr Afzal has redoubled his criticism in an explosive tweet, saying for the first time – after “30 years working in criminal justice” – that the nation’s leader was “lying”.
“Having overseen prosecution of perhaps a million cases worked with dozens of ministers of all parties I have never felt need to say this: This Prime Minister (& Home Secretary) are both lying to us,” he warned.
But, on the BBC, the prime minister claimed that Khan had been on the streets, able to kill, because of a “leftie government”.
“His release was necessary under the law because of the automatic early release scheme under which he was sentenced,” he insisted.
“That was the reality, and that was brought in by Labour with the support of Jeremy Corbyn and the rest of the Labour Party.”
During a bad-tempered interview, the prime minister revealed there were about 74 other people convicted of terror offences who had been released early in similar circumstances to the London Bridge attacker.
He insisted he bore no responsibility for past Conservative spending cuts, saying: “I’m a new prime minister – we take a different approach.”