The UK's terrorism watchdog renewed calls for extremists to be given lie detector tests, arguing that deradicalisation programmes do not work.
Jonathan Hall QC, the UK's independent reviewer of terrorism legislation, said there was "no magic bullet or special pill" that could help to change convicted extremists.
Mr Hall told The Times that schemes should be offered in conjunction with heavy supervision and said that the public should be under "no illusion" that they would be effective.
“There is no magic bullet, there is no special pill you can take that deradicalises people, whether they’re coming back from overseas from Syria or whether they're being released from prison," he said.
"It's a pretty difficult, complex and fraught process. You can’t tell the public that you can place someone with a theological mentor ... and they’ll come out the other side. It’s far more difficult than that.
“I can see why people try, because if you didn’t try, it would be throwing away all hope, and these offenders are also subjected to some pretty major restrictions, so it’s worth giving them an opportunity to change. And there will be some who will change, but you should be under no illusions. It is not some automatic process. And in many cases it simply won’t work. It doesn’t mean it’s not worth trying.”
This year, Mr Hall published a review into last year's terrorist attack near London Bridge in which two people were killed, and made 45 recommendations.
In it, he recommended that terrorist offenders should take lie detector tests. The Home Office is still considering his report.
Cambridge University graduates Jack Merritt, 25, and Saskia Jones, 23, were fatally stabbed by Usman Khan during a prisoner rehabilitation programme held near London Bridge in November 2019.
Khan, 28, was out on licence when he attended the event organised by the university after being released early from a prison sentence for plotting to bomb the city’s stock exchange.
Mr Hall's comments come as the UK government appeals against the return to the UK of Shamima Begum, the London schoolgirl who left to join ISIS and was stripped of her British citizenship.
A debate has been ongoing over whether she should be allowed to return and receive deradicalisation support.
In a second report, published in March, Mr Hall made 28 recommendations to Home Secretary Priti Patel to overhaul the UK’s terrorism laws.
Recently, Ms Patel said she had rejected 15 of them, that the government was still considering three, and had partially accepted another.
One of his recommendations on Temporary Exclusion Orders has been accepted by Ms Patel.
TEOs are used as a tool by the UK to deal with terrorist fighters and suspected terrorists returning to the country from abroad.
They enable the government to take away their passports upon their return to the UK, impose strict monitoring conditions on them and require that they attend deradicalisation schemes.
Mr Hall had pointed to a gap in the law that resulted in two-year TEOs being imposed on people before they returned, which would then expire within months of them being in the UK. This meant the courses did not have time to be effective. He said the orders should start when people landed in the UK.
Ms Patel also agreed to consider imposing TEOs on non-British citizens.
The UK runs the anti-radicalisation Prevent programme, which offers practical help to prevent individuals from being drawn into extremism, and the Channel programme, which intervenes in cases where someone is already on that path.
Figures released last month showed Islamist radicalisation referrals to Prevent have risen for the first time in four years.
Prevent, which was launched to monitor potential terrorist activity in the UK, reported a 10 per cent increase in cases flagged to the authorities between April 2019 and March of this year.
When authorities decide there is a risk that a person referred to Prevent could be drawn into terrorism, they are assessed as part of the Channel scheme and potentially taken on as a case. Engagement with the scheme is voluntary and it is not a criminal sanction.
Channel recorded its highest yet number of referrals over the past year, with 697 of the 1,424 referrals examined taken on as cases.