Violent extremism is “clearly not under control” in jails and there is an “institutional squeamishness” among Government officials to tackle the problem, a former prison governor has claimed.

Ian Acheson, who led an independent review of Islamist extremism behind bars in 2016, made the comments after it is feared a convicted killer allegedly attempted a jihadist attack on prison guards at HMP Winchester.

An improvised weapon in the shape of a blade was found on the prisoner, who is not behind bars for terrorist offences, during a search after the incident.

The PA news agency understands he was afterwards overheard by officers shouting to a fellow inmate in a nearby cell that it was “a J thing” – which it is believed officials are taking to mean jihad or jihadi.

No staff were injured and the incident is not currently being investigated as terror-related.

The news comes amid growing concerns over radicalisation behind bars in the wake of three terror attack in as many months – one of which took place inside a jail while the other two involved convicted terrorists recently released from prison.

Speaking on Radio 4’s World at One programme, Mr Acheson said violent extremism was “clearly not under control inside prisons” and “presents a lethal threat” both inside and outside jails.

He said prison staff are “increasingly being confronted with a lethal extremist-inspired threat for which they are patently not equipped to deal with”, adding: “I think there is an institutional squeamishness at the top of the prison service to deal with this problem which unfortunately has now led to the situation that we see where we have got violent extremism that is at best delayed inside custody and at worst weaponised.”

Meanwhile Jonathan Hall QC, the independent reviewer of terrorism legislation, suggested keeping prisoners behind bars for longer could “expose them to worse influences” than if they were released.

In the published analysis on the emergency laws the Government has proposed to keep terrorists behind bars for longer, he warned the changes could create a “cliff edge” when offenders are instead freed without any restrictions at the end of their sentence.

There had been “foot dragging” over bringing in changes to tackle the problem, Mr Acheson claimed, adding that he was not convinced all recommendations for improvements he had made – such as ensuring prison imams had faced security checks – had been implemented.

Xeneral Imiuru had been segregated in the prison over concerns of violent behaviour towards staff, PA understands.

It is thought prison officers attended his cell in the segregation unit to check on his condition after he was seen lying on the floor.

But a struggle took place after he was found to be conscious.

The 20-year-old, also known by the surname Webster, was the first person to be convicted of an acid attack killing and was jailed for 17 years in 2018.

He pleaded guilty to the manslaughter of Joanne Rand, a 47-year-old carer for dementia patients.

The innocent bystander had been visiting her daughter’s grave when she was hit with the high-strength sulphuric acid which Imiuru had armed himself with and was knocked out of his hand during a row with another man nearby.

The mother-of-three had 5% burns on her body and died 11 days later from multiple organ failure after contracting septicaemia.

A Prison Service spokesman said: “Police are investigating an incident at HMP Winchester on February 13.

“Violence against our hard-working staff will not be tolerated and those responsible will face tough punishment, including more time behind bars.”

The POA, formerly known as the Prison Officers’ Association, called for the investigation to be carried out “quickly” and said questions were being asked over how inmates might have access to weapons, particularly in segregation.

Hampshire Constabulary said an investigation was in its early stages as the incident was only reported to police on Tuesday.

It is understood officials contacted the force on Monday after the incident was initially reviewed by an internal adjudicator who deals with prison discipline matters.

Earlier this month Sudesh Amman wore a fake suicide belt as he grabbed a knife from a shop in Streatham High Road, south London, on Sunday, before stabbing two bystanders.

The 20-year-old had been jailed for possessing and distributing terrorist documents in December 2018, but was freed automatically halfway through his sentence less than a fortnight ago.

He was put under 24-hour police surveillance on his release after it is understood security services regarded him as an “extremely concerning individual”.

Two inmates wearing fake suicide belts stabbed a prison officer at maximum security jail HMP Whitemoor in January.

Saskia Jones and Jack Merritt were killed by Usman Khan in November when he launched his attack armed with two knives and wearing a fake suicide vest during a prisoner rehabilitation programme near London Bridge – nearly a year after he was released halfway through a 16-year jail sentence for terror offences.

A German Islamist preacher was sentenced on Wednesday to 5-1/2 years in prison on four counts of supporting a foreign terrorist organisation.

Prosecutors said Sven Lau served as an “extended arm” of the Jaish al-Muhajireen wal-Ansar, an insurgent group made up of around 1,500 Chechen, Uzbek and Tajik fighters, which in 2015 pledged allegiance to the Nusra Front, then an al Qaeda affiliate. They said he collected funds and recruited fighters for the group, which is listed as a terrorist organisation in Germany.

Lau, a hardline Muslim preacher from Germany’s western state of North-Rhine Westphalia, had also turned up in the city of Wuppertal with nine supporters in tow, all wearing orange vests emblazoned with the words ‘Sharia Police’.

Prosecutors said this was an illegal gathering as he and eight others were trying to recruit others to join them.

Prosecutors had called for a 6-1/2 year prison sentence for Lau, while the defence had pleaded for an acquittal.