Terrorists are able to network and radicalise other inmates while in high-security jails because of inaction by the prison service, it has emerged.
There are currently more than 200 people jailed for terror offences in Britain, but hundreds more have been flagged as potential extremists and prison officers fear the real number is far higher.
But efforts to monitor their activity rely on stretched staff whose training was deemed “completely inadequate”, flagging inmates to a specialist unit.
The warning comes after two prisoners wearing fake suicide vests launched a suspected terror attack at HMP Whitemoor.
The previous day, a court heard how a man who attacked police officers with a sword outside Buckingham Palace got tips on avoiding conviction from terrorists while being held at HMP Belmarsh.
And in November, the London Bridge terror attack was launched by an Islamist who underwent deradicalisation programmes while serving a prison sentence for another plot.
The Ministry of Justice says prison officers are trained to report suspicious behaviour, but a union called the training “completely inadequate” and said staff are too stretched to deal with radicalisation amid a crisis of drugs and violence.
Mick Pimblett, assistant general secretary of the Prison Officers’ Association, said new staff were given one-day extremism training during their initial course but that he never received it during his 30 years of service.
He told The Independent that prison officers in high-security prisons were more “clued up” than colleagues in category B and C jails that inmates are moved into before their release.
“The officers aren’t dealing with it on a daily basis so there could be problems,” Mr Pimblett added.
“Staffing levels are an issue as well. The staff are stretched to the point they don’t have time to do anything, they are doing their best to run a regime.
“They can’t go around getting intelligence from prisoners.”
The former prison officer, who left the service in November 2018, said that staff may not be able to understand discussions by inmates speaking other languages such as Urdu and Arabic.
According to the most recent statistics, there were 224 people in custody for terrorism-related offences in the UK, with three-quarters classified as Islamists and 17 per cent far-right extremists.
But there are between 500 and 800 inmates being managed under a “counterterrorism specialist case management process” at any one time.
A prison officer working in the high-security estate told The Independent even that figure was “nowhere near” the real number of Islamist extremists inside.
“You’ve got someone who is in for five or six years, an ordinary criminal, and all of a sudden they are radicalised into hating anything about the west,” he added.
Speaking on condition of anonymity, he said Friday prayers were used as a means to network and officers “can’t do anything about it”.
“There are lots of people who are more of a threat when they leave than when they come in,” he added. “Most of them will revert back to their normal way of life but some will be so radicalised during their time in prison that they will want to go out and attack the west.”
Concerns have repeatedly been raised about radicalisation inside jails, with official inspections warning of the existence of Muslim “gangs” and terrorist prisoners being an “adverse influence on others”.
The Westminster attacker, Khalid Masood, is among the terrorists believed to have converted to Islam while in prison, while other Muslims have allegedly been radicalised on the inside.
A 2016 review of Islamist extremism in prisons recommended the creation of separation centres for the most dangerous inmates, but one of the three units was emptied and closed in December.
Ian Acheson, a former prison governor who led the probe, said there were still “abundant opportunities for networking and radicalisation”.
“Terrorist prisoners who are outside separation units are free to associate with non-extremists if they are in normal locations,” he told The Independent.
“This means in security terms, that they have ready access to often highly violent and vulnerable young men in search of meaning and excitement.
“Whether and to what extent they are monitored or potentially subversive activities noticed depends to a large extent on staff being available, skilled and capable to submit security intelligence reports, and those reports being actioned further up the operational chain.”
The Ministry of Justice said extremism concerns are reported to counterterrorism specialists, who are part of a joint extremism unit involving the prison service, intelligence services and probation.
The Home Office and Cabinet Office review how specialists manage extremists but their findings are not publicly published.
However, recent revelations suggest the system is failing. Usman Khan, 28, launched the London Bridge terror attack less than a year after being released from prison on licence, after being jailed over a previous bomb plot.
He had been held at the same two jails that were also home to Brusthom Ziamani, who had planned to behead a British soldier in 2015.
He was reportedly able to hold “Sharia courts” in his own cell and radicalise fellow inmates, potentially including the man who joined his alleged terror attack in HMP Whitemoor on 9 January.
Mohiussunnath Chowdhury was held at the London prison on remand from August 2017 to December 2018 after attacking police officers with a sword outside Buckingham Palace.
He allegedly told undercover police officers how fellow inmates gave him tips to avoid conviction at a retrial where he was acquitted, and that he had “learned a lot from likeminded brothers whilst in prison”.
Mr Chowdhury is currently on trial accused of planning new attacks after his release.
In 2017, three terror convicts known as the “three musketeers” planned a new attack together after being allowed to mingle in prison.
A Ministry of Justice spokesperson said: “More than 29,000 prison officers have received enhanced extremism awareness training to spot signs of radical behaviour in our jails.
“They provide vital intelligence to our network of counterterrorism specialists which monitors and disrupts suspected extremists throughout the prison and probation service.
“We’ve also recruited 4,400 more prison officers in the last three years and are spending and extra £2.75bn to transform our jails and enhance security.”