Britain is not the first country to consider holding the highest-risk Islamic extremist prisoners in specialist isolation units.

In January 2015, when 17 people were killed in attacks in Paris, the Socialist government in France vowed to act on what it called the “major issue” of radicalisation in prisons.

Chérif Kouachi and Amedy Coulibaly, two of the men who carried out the attacks, including the storming of the offices of Charlie Hebdo magazine, shared the same hardline mentor when they were at Fleury-Mérogis prison near the French capital in 2005.

In the aftermath of the attacks, France’s prime minister, Manuel Valls, announced that Islamic extremists would be isolated from the rest of the prisoner population to prevent jails from being used as a breeding ground for radicalism.

This measure “must become widespread” but “it must be done discerningly and intelligently,” Valls said.

Months earlier, Fresnes prison, 7 miles (11km) south of Paris, began a scheme to single out extremists who were recruiting fellow prisoners and keep them apart from others for most activities. Early this year, five projects were set up to provide separate dedicated units for a number of radicalised prisoners in detention in France. De-radicalisation projects were then put in place on the units.

The policy has since been the subject of debate, with a report released in June by the French prisons auditor critical of what it called “disparate” and tentative measures hastily put in place, which offered “unsatisfactory” responses to the unprecedented phenomenon of radicalisation in French prisons.

Questions remain as to whether the handful of special prison units that have been opened can be rolled out across France. Before the units were created, the former justice minister Christiane Taubira expressed “serious reservations” over the idea of grouping extremists together, with some magistrates fearing that it could create concentrated networks of jihadis.

The French prisons controller, Adeline Hazan, said rolling out the project across France was not currently “realistic” given the pressing parallel problem of serious overcrowding in France’s jails and the “spectacular” rise in the number of people implicated in extremist networks related to Iraq and Syria, as well as those on remand for terror offences.