Seven hundred prisoners have been identified as a potential extremism risk, it has emerged.
The figure, published for the first time since authorities launched a major crackdown on radicalisation behind bars, includes inmates flagged up over concerns they hold extreme Islamist or far-right beliefs.
It was disclosed as the Government announced a second separation centre to house the "most subversive" prisoners has opened at HMP Full Sutton in Yorkshire.
Ministers announced plans to set up three specialist facilities to isolate fanatics from the rest of the population last year.
Moves to establish the "jails within jails" gathered pace after an official review warned that Islamist extremism was a growing problem in prisons in England and Wales.
The assessment found evidence of offenders advocating support for Islamic State and "charismatic" prisoners acting as "self-styled emirs" to radicalise other inmates.
The first separation centre opened at HMP Frankland in Durham in July.
Announcing the opening of the facility at HMP Full Sutton, the Ministry of Justice said: "With 700 prisoners considered a risk due to their extremist views, and foreign fighters returning from Syria and Iraq hardened and dangerous, the Government is meeting the challenge of confronting and countering the spread of poisonous ideology within prisons."
It is understood the 700 figure is an overall estimate of all inmates linked to any form of extremism, including Islamist or far-right ideologies.
The MoJ said there has been a 75% increase in prisoners convicted of terrorism-related offences in the last three years.
At the end of December there were 224 individuals in custody in Britain after being charged with or convicted of terrorism-related crimes.
Of those, 192, or 86%, were assessed as holding Islamist views, while 21, or just under a tenth, were identified as having extreme far-right sympathies.
Other prisoners held for non-terrorism offences but deemed to be an extremism risk were also counted in the 700 figure.
Justice Secretary David Gauke said: "As a result of the Government's unprecedented action to protect the public from extremists, we have seen a 75% rise in terrorism-related prisoners over the last three years.
"That means we need to do more than ever before to confront and counter the threat, including the spread of all forms of poisonous ideology within prisons - and we are meeting that challenge.
"With thousands of prison staff now trained to deal with extremism, an enhanced intelligence capability and separation centres for the most subversive prisoners, we are well equipped to deal with this threat."
Inmates can be moved to the specialist units if they are linked to terror plotting or considered to pose a risk to national security.
Those seeking to influence others to commit terrorist crimes, or whose extremist views are undermining good order and security behind bars can also be taken out of the mainstream population.
The third centre will be in operation by the end of the year, the MoJ said. The three sites combined will hold up to 28 individuals.