An inmate suspected of attacking a prison officer is understood to be in jail for "preparing an act of terrorism". Brusthom Ziamani was found guilty in 2015 of planning to behead a soldier.
A union says the officer, who was seriously injured, was attacked from behind by two inmates.
So who are the people in prison for terror offences?
How many people are in prison for terror offences?
The majority are Islamist extremists, although their numbers fell to 173 last year - down from 187 in 2017.
There has been a rise in the number of far-right extremists jailed, with an increase from six to 38 over the past five years.
Overall, there are 224 people in prison for terror-related offences in Great Britain.
What offences are they guilty of?
Specific anti-terror laws include offences like preparing an attack, funding a terrorist group and attempting to influence radicalisation.
Of those convicted last year, the most common offence (in 11 cases out of 56) was membership of a banned organisation. This was followed by "preparation for terrorist acts" (nine convictions).
People can also be convicted of crimes which aren't terror-specific, like murder, or public order offences, which can carry a heavier sentence if they are terror-related.
To find someone guilty of "preparation of terrorist acts" it has to be proved they had the "specific intent to commit an act or acts of terrorism".
It can include everything from having a minor role in a plot to planning multiple murders, or being in possession of bomb-making equipment.
Who is convicted of terror-related offences?
A large proportion of convicted terrorists are young and male.
The government's counter-terror strategy notes that extremist groups "cynically groom the vulnerable and the young to join their movement".
What are the government's plans?
The government is planning to toughen sentencing laws for terrorists. It has said it wants to see minimum 14-year sentences for "serious terror offences" and that those convicted should serve all of that time in prison.
Currently, most people convicted of terror acts receive less than a 10-year sentence and most will not serve all of it in jail.
For almost all prison sentences (terror-related or not), some of the time will be spent outside jail in order to allow some rehabilitation in the community.
The prime minister sought to change prison sentences for those convicted of "serious terror offences" after it emerged that the London Bridge attacker, Usman Khan, had been released halfway through his sentence, without the approval of a parole board.
Part of the reason was because a number of changes had been made to the rules around prisoners' release dates. However, the most recent approach to the release of serious criminals in 2012 would have required him to spend longer in jail and face the parole board.
What the government considers to be a "serious terror offence" is yet to be revealed.
What's already being done in prisons?
A specialist unit was set up by the Home Office and HM Prisons and Probation Service in 2017, to tackle extremism in prisons.
It trains staff in how to deter offenders from being radicalised and advises jails on how to deal with dangerous prisoners.
Last year, prisons minister Lucy Frazer said more than 19,500 prison staff had received specialist extremism awareness training.
What else is being done to prevent terrorism?
Part of the government's strategy for preventing terrorism includes a duty for hospitals, schools and local authorities to safeguard against extremism.
The programme, called Channel, can lead to individuals receiving support to counter extremism. However, it has been criticised for targeting particular communities.
In recent years, the number of people receiving Channel support increased, with as many people now thought to be at risk of converting to right-wing extremism as to Islamist extremism.