The architect of the government programme for moving convicted terrorists from prison into the community believes the current system lacks the “legitimacy and credibility” required to rehabilitate extremists safely.

Simon Cornwall, who set up the probation service’s central extremism unit, said that as a result of “a dumbing down of how things are done”, the current approach was missing the safeguarding and human relationships required to modify behaviour and reduce risk.

His intervention follows the attack at London Bridge by convicted terrorist Usman Khan, who was out on licence from prison when he killed Jack Merritt, 25, and Saskia Jones, 23, and injured three others during a meeting of the Cambridge University rehabilitation initiative Learning Together on 29 November.

Cornwall, who worked with a number of the nine-strong terror group – including Khan – which was jailed for plotting to bomb the London Stock Exchange in 2010, said: “There’s been a breakdown in the system, a dumbing down of how things are done.

“The criminal justice system has become very insular, moving away from partnerships with community groups who can form crucial relationships with offenders to a really securitised view. It has lost the legitimacy and credibility it had before,” he said.

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Cornwall, who introduced the concept of mentors working with terrorists to help reintegrate them into the community, said that such a “hands-on” human approach had been jettisoned in favour of a reliance on technology such as tagging.

Khan had been wearing an electronic tag, managed by the probation service and subject to low-level monitoring, when he embarked on his stabbing rampage.

“At one point Khan would have had a mentor who would have gone with him to London, who knew him intimately and would have seen the change in him. It’s very difficult to sustain a lie,” said Cornwall. “Probation is about relationships and there wasn’t a close enough relationship for someone to say: ‘Hold on, something’s going on here’.”

The central extremism unit, responsible for convicted terrorists who are released from prison on licence, was set up by Cornwall in 2008 and funded by the office for security and counter-terrorism (OSCT), which operates under the control of the Home Office and brings probation together with the intelligence agencies and police.

Cornwall, also a strategic lead for the Home Office’s counter-extremism initiative Prevent until leaving in 2015, also questioned the political response to the London Bridge attack.

Following the incident, prime minister Boris Johnson announced that those convicted of the most serious terrorist offences should never be released from prison, prompting anger that politicians were exploiting the tragedy.

Facebook Twitter Pinterest Jack Merritt, one of the London Bridge victims, was working on a rehabilitation initiative when he was killed. Photograph: Jack Merritt/Instagram

Cornwall, a fellow at the German Institute on Radicalisation and Deradicalisation Studies (Girds), said such an approach ignored the fact that individuals are vastly different and required bespoke approaches. “To say that they need to be locked up for ever shows no understanding whatsoever of what goes on,” he said.

Rather than locking up offenders, he advocated hiring workers who followed the Salafist branch of Islam to work to help deradicalise extremists.

Otherwise, said Cornwall, it could be similar to “having a heroin addict and to help them you give them someone who’s been addicted to aspirin. They need someone who’s been on the same journey.”

Hanif Qadir, a former Home Office senior counter-radicalisation expert, also criticised the disengagement of the government from community groups and clerics who could help convicted terrorists safely reintegrate. “They’ve dismantled the system, closed down external services that developed close relationships that could successfully rehabilitate some of the most hardened offenders,” he said.

The fallout from Khan’s attack has led to calls for an independent review. Since the attack, it has emerged that he attended two counter-terrorism programmes that had not been fully tested to see if they were effective and wrote a letter from jail asking to take part in a deradicalisation course to become “a good British citizen”.