It is a “problem” that three convicted terrorists were allowed to associate behind bars and form a gang, a terror watchdog has said.

Hashem Abedi, brother of the Manchester Arena bomber, Parsons Green Tube bomber Ahmed Hassan, and Muhammed Saeed – who spoke about carrying out a knife attack in London – were found guilty of attacking a prison officer in Belmarsh’s high-security unit in May 2020.

Jonathan Hall QC, the independent reviewer of terrorism legislation, said the case demonstrated an attempt by an “established gang” to seize power from authority, and suggested how measures like separation centres could be used to keep terrorists away from each other in custody.

Abedi – who conspired with his elder brother, suicide bomber Salman Abedi, 22, who killed 22 people and injured hundreds more at the Manchester Arena on May 22 2017 – was suspected of being the “amir” or “leader” of a group of Islamist terrorist inmates inside Belmarsh’s “prison within a prison”, Woolwich Crown Court heard.

The trio were involved in a fight with a group of non-Muslim prisoners in the months before the attack, when their incentive and earned privileges (IEP) level was downgraded from “standard” to “basic”.

Speaking to the PA news agency, Mr Hall said: “Yes, I think it’s a problem that they were allowed to associate in a way that led to the formation of a gang.

“There’s territorial behaviour here. So, not just the attack on the prison officer … but there was an orchestration of complaints and there was an attempt to get the regime changed on behalf of him and his brothers. And brothers here means fellow Islamist gang members.

“So this case demonstrates an attempt by an established gang to seize power from the authority.”

Asked how terrorists can be prevented from colluding behind bars, Mr Hall said: “In terms of what you do about it, I think you’ve got to recognise that these are not just gangs like any other gangs.

“This is a gang led by a terrorist mass murderer and the consequences of this sort of gang led by this sort of individual having influence within the prison are so serious that it has to be disrupted – in terms of the impact on other prisoners, recruitment and radicalisation within prison, and potential encouragement to further offences against the authorities.”

He said the authorities should be “praised” for bringing the case to court as it is a “very clear and public record of what they have done and it will enable the authorities to manage these individuals in future with greater confidence”.

This, for example, could include other measures such as using separation centres, he added.

Mr Hall has been reviewing terrorism in prisons in England and Wales and his findings are anticipated to be published in the coming months.

Ian Acheson, a former prison governor who carried out a Government-commissioned review of Islamist extremism in jails, told The Independent the incident was a “huge security failure that ought to be a shocking wake up call for ministers”.

The Ministry of Justice said it had zero tolerance for any type of radicalisation activity in custody, prison staff are trained to spot and deal with extremist views, and separation centres are available to hold the most dangerous offenders.

A spokesman said: “It is absolutely right that terrorist offenders of such risk are housed in our most secure prisons so they can be subject to much stricter monitoring and conditions, including rigorous oversight of who they are associating with, routine checking of correspondence, and higher staffing.

“We will never tolerate such appalling attacks against hard-working staff and – as in this case – will always push for the strongest possible punishment.”