The government has launched an urgent review of terrorism offenders who are imminently to be released from prison, amid questions over MI5 monitoring.

The London Bridge attacker, Usman Khan, was among more than 500 terrorist prisoners freed from British jails since 2011.

Robert Buckland, the justice secretary, said the government was reviewing the restrictions applied to 74 released extremists who are currently under licence conditions.

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“We will look at a wider list of a few hundred people who might not have committed terror offences, but are known to have extremist views,” he told LBC radio.

“I want to make sure that we cover this thoroughly, and we also look ahead to people who might be released from custody in the next few weeks.”

At least one man, Nazam Hussain, has so far been arrested as a result of the Ministry of Justice’s review.

The 34-year-old was detained on suspicion of preparing terrorist acts and recalled to prison.

Official figures show that of the 223 terrorist prisoners in custody as of March, 80 per cent were Islamists and 15 per cent were extreme right-wing.

The figure has been at a historical peak since 2018, and includes people in custody for terror offences and extremists jailed for other crimes.

Mr Buckland said he was working to ensure that when they are released, their “licence conditions are as stringent as possible”.

Khan was under 22 such conditions limiting his activities, movement and associations, but was still able to launch his attack at a rehabilitation event on Friday.

Officials insist that he appeared to comply with deradicalisation programmes after being imprisoned for his part in a plot to bomb the London Stock Exchange in 2012.

Sources told The Independent that Khan, a former member of Anjem Choudary’s al-Muhajiroun network, was being monitored by MI5 but not intensively because there was “nothing to indicate that he was seeking to carry out an attack”.

The admission raises questions over MI5’s assessment system for potential lone terrorists, which was previously criticised following the murder of Lee Rigby in 2013 and the 2017 London Bridge attack.

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In a report released last month, the chief coroner of England and Wales described the system as “imprecise and highly variable in its assessments”.

“In my opinion, there is a risk that future deaths could occur unless action is taken,” said Mark Lucraft QC.

Lord Anderson, the former Independent Reviewer of Terrorism Legislation, had also called for improvements after assessing the intelligence held on the Islamists who launched a spate of attacks in 2017.

And three years before, parliament’s Intelligence and Security Committee had warned of “gaps” over terrorists acting alone and attempting uncoordinated and low-tech plots.

Raffaello Pantucci, a senior associate fellow at the Royal United Services Institute, said security services had struggled to adapt to the shift from organised terrorist networks to “lone actors”.

“That’s the shape of the threat that we face and I think its unpredictable nature is part of its problem,” he told The Independent.

“MI5 have developed a lone-actor tool that I suspect is usually very good, but not perfect.”

The 2017 London Bridge attacker Khuram Butt had initially been assessed by the tool as only having a “weak capability” to attack and it is likely that Khan had been graded.

Mr Pantucci said that even if Khan had genuinely engaged with deradicalisation programmes, he may have had a more recent “personal crisis” that sparked his attack.

“Obviously whatever engagement happened hadn’t been maintained,” he added.

Usman Khan, 28, who was shot dead by police after stabbing a man and a woman to death and injuring several others on London Bridge, 29 November 2019. (West Midlands Police via AP)

“It does raise questions about the resource that would be required to sustain engagement on an individual level with a whole cohort of people, which might be needed for decades.”

The Independent understands that Khan was subject to the highest level of multi-agency public protection arrangements, involving police and probation officers.

He had been jailed in 2012 as one of a nine-man terror cell who plotted a bombing attack on the London Stock Exchange.

Khan was originally handed an indeterminate prison sentence for public protection with a minimum term of eight years. 

But the sentence was quashed on appeal in 2013 and he was given a determinate 16-year term, meaning he was released automatically when he reached the halfway point last December.

The Court of Appeal’s ruling said Khan and his co-defendants had “fallen under the influence of radical or extremist clerics” and became involved with al-Muhajiroun.

Judges described how Khan had discussed building pipe bombs and was attempting to radicalise others and set up a jihadi training camp in Pakistan.

A paper published by the Commission for Countering Extremism in October warned that al-Muhajiroun activists could “hold steadfast to their beliefs”.

“They regard any compulsory deradicalisation programming with deep suspicion, as an attempt by the government to control their thoughts and compel them to accept a ‘correct’, state-sanctioned understanding of Islam,” Michael Kenney wrote. 

“They see their forced participation in such programming as another form of repression, to go along with the raids, arrests, and other controls they have already endured.”

Khan launched the knife attack at a prisoner rehabilitation conference, associated with Cambridge University’s Institute of Criminology, where he was a guest.

One of the victims, Jack Merritt, was a coordinator of the Learning Together programme, while fellow victim Saskia Jones was a volunteer.

Khan launched the attack without warning at around 2pm on Friday, and was fought by delegates and staff at Fishmongers’ Hall before being chased on to London Bridge and shot dead by armed police.