The psychologist behind the UK’s main deradicalisation programme for terror offenders has warned it can never be certain that attackers have been “cured”.

Christopher Dean said some terror offenders who take part in his Healthy Identity Intervention (HII) scheme appear to regress due to complex reasons such as who they mix with.

Mr Dean’s comments come after HII participant Usman Khan stabbed two people to death near London Bridge on November 29.

Khan was a convicted terrorist who had been a member of an al Qaida-inspired group that plotted to blow up the London Stock Exchange.

The HII scheme involves offenders like Khan attending sessions with a psychologist who encourage them to talk about their motivations, beliefs, identity and relationship with society.

Former senior Home Office official Ian Acheson said attention was drawn to shortcomings of the HII programme in 2016.

Forensic officers at the scene of the terror attack on London Bridge
Forensic officers at the scene of the terror attack on London Bridge (Kirsty O’Connor/PA)

Speaking on BBC Radio 4’s Today programme, Mr Dean said individuals can both progress and regress under healthy identity intervention.

“Sometimes people move up two rungs, sometimes individuals may say I’ve had my doubts about this or that and they may be willing to speak to people, but equally they may go down rungs as well.

“They may come into contact with individuals, they may go through a spell in life where they may feel let’s say aggrieved again, where they may begin to re-engage with groups or causes or ideologies associated with their offending behaviour,” he said.

Mr Dean said some offenders he worked with needed 20 or more sessions to show signs of positive change.

“We see some individuals who may have been part of a group for many years or have been invested or identified with the cause for many years. [Leaving that group] is an incredibly difficult thing to do,” he said.

He added that there is no guarantee of success.

“I don’t think you can ever be sure that that’s occurred. I think you can get increasing evidence over time and particularly behaviourally when people begin to behave in different ways and that’s consistent over time and in different places.

“People can get more reassured and confident about change and progress that people are making, but yeah, I think we have to be very careful about saying someone has totally changed or has been cured.”

Mr Dean added that he would have a “healthy scepticism” of the notion that there is a perfect system to deradicalise terrorists.

He said: “I think we need to be careful about suggesting that interventions in themselves are the solution or the only solution or psychology is, but I think it’s about continuing to work together in our learning.”

Mr Acheson, who carried out a government review of extremism in prisons, said HII is “a psycho-social intervention that ignores or cannot really accommodate the theological foundations for violent extremism”.

Mr Acheson added that evidence from people close to offenders who had been through HII in 2016 shows it appeared “easy to game or manipulate”.

He said: “In other words there was a degree of false compliance with the healthy identity intervention course which meant that it would appear that people had improved or made progress when in fact they might simply have been disguising their intentions in order for attention to pass on to someone else.

“I think we must accept that there may be a small number of people who are potentially ideologically  bulletproof and do not wish to recant their hateful views and we must actually start looking at them through the lens of public protection and national security rather than perhaps create unrealistic expectations of rehabilitation.”

He added that it is better to “apply an individualised treatment plan to offenders to tackle their unique needs”.

“Of course doing something like that takes a long time, it takes a lot of skill and effort and expertise and it will be extremely expensive.

“But if you look at the potential lethality of the threat, which has been demonstrated by Usman Khan’s murderous rampage, I think society does expect and the public do expect that they are protected by the most extensive means that we have available to intervene with individuals and look at their individual pathologies and how they arrived at their violent extremism as a means to try and get them out of that mindset,” he said.

Usman Khan killed two people and injured three others in a knife rampage before being shot dead by police in November.

He had been released from prison on licence in December 2018, by which time Khan reportedly appeared to be responding to rehabilitation.