Keeping people who are serving a controversial indefinite sentence beyond their minimum term in prison has cost the taxpayer more than £1bn since 2012, Sky News analysis has found.
Sky News has analysed data relating to those serving imprisonment for public protection (IPP) sentences.
This is a type of open-ended sentence that was abolished more than a decade ago following widespread concern over its implementation and psychological impact on inmates.
Anyone jailed on an IPP can only be released once they have served their tariff after the Parole Board has given its approval.
From April 2012 to December 2023, it cost the taxpayer an estimated £1.1bn to house unreleased IPP prisoners who were serving time beyond their original tariff - the term used to describe the minimum amount of time they had to spend in custody before they could be considered for release by the Parole Board.
It does not include those who have been recalled back to prison after initially being released under strict licence conditions.
The analysis comes as new data released by the Ministry of Justice (MoJ) shows 2,852 people remain incarcerated under the IPP system, including those who have been recalled back into custody.
Of this number, 1,227 have never been released, while 1,210 are behind bars beyond their original tariff.
In the analysis, Sky News took the average cost of a prisoner in each financial year back to 2012/13 and multiplied that by the number of prisoners remaining on expired IPP sentences at the end of each year.
Adding together these costs for each year since 2012/13 comes to a total of £1.1bn.
What are IPP sentences?
Implemented in 2005 under the then Labour government, IPP sentences were intended for the most serious violent and sexual offenders who posed a significant risk of serious harm to the public but whose crimes did not warrant a life term.
Although the government's stated aim was public protection, concerns quickly grew that IPP sentences were being applied too broadly and catching more minor offenders - with many serving time in prison much longer than their initial term.
In light of the criticisms, IPPs were scrapped in 2012 but the change was not applied retrospectively - meaning those who were in prison at the time the sentence was abolished were not able to benefit.
There have long been calls from campaigners for the government to review the use of IPP sentences - which have been described as "cruel and degrading" by Alice Jill Edwards, a torture expert at the UN.
What's the alternative?
In 2022, parliament's Justice Select Committee published a report recommending a resentencing exercise, arguing the IPP sentence was "irredeemably flawed" and caused "acute harm" to those serving them due to the prisoner not knowing when they might be released.
Successive governments have been reluctant to carry out a resentencing exercise on the grounds it could compromise public safety.
However, Justice Secretary Alex Chalk - who has described IPPs as a "stain" on the justice system - has come under pressure to heed the committee's calls due to the current capacity constraints in the prison system.
Prison estate approaching capacity
Data from the MoJ published last week shows the prison estate in England and Wales is approaching its operational capacity limit, with just 1,430 spaces left.
According to the campaigning organisation the Institute of Now, the number of people still inside on IPP sentences beyond their tariff could fill the equivalent of more than four average-size UK prisons.
Henry Rossi, a human rights campaigner and founder of The Institute of Now, said: "Far too many people, both prisoners and their families, have been subjected to psychological torture from this wicked sentence, which in so many cases, has led to suicides.
'Blood on it's hands'
"Prisons are not the place to manage those that have served their time as punishment. The UK has blood on its hands and the government must urgently relook at this draconian sentence and release post-tariff IPP prisoners with the appropriate support."
A MoJ spokesperson said: "We have reduced the number of unreleased IPP prisoners by three-quarters since we scrapped the sentence in 2012, with a 12% fall in the last year alone where the Parole Board deemed prisoners safe to release.
"We have also taken decisive action to curtail licence periods and continue to help those still in custody to progress towards release, including improving access to rehabilitation programmes and mental health support."
The MoJ figures and Sky News analysis comes as a separate report from the Independent Monitoring Board (IMB) shows how care and separation units (CSUs) are regularly being used as a "stop gap" to manage prisoners with severe mental health needs - including those serving IPP sentences.
Mental health 'dramatically deteriorated'
In one case, a man with a schizophrenia diagnosis cumulatively spent almost nine months in the CSU - also known as segregation or "the seg" - during which time the IMB said his mental health "dramatically deteriorated".
In another example, an IPP prisoner with a personality disorder spent over 800 days in the CSU before they were transferred to an appropriate unit.
Elisabeth Davies, IMB national chair, said: "Segregating any prisoner, especially those with mental health needs, is not a decision taken lightly by prison managers. While they clearly feel that they have no other option, CSUs should not be used as holding bays for these vulnerable individuals.
'A disturbing picture'
"While local IMBs found that overall staff worked hard to support prisoners in the best way that they could, collectively they have reported a disturbing picture of mentally unwell men spending lengthy periods in isolation, which often results in a deterioration of their mental health.
"If anything is to change, the solution is, and has always been, providing appropriate mental health provision in the community and tighter controls around the transfer times from prison to hospital."
A government spokesperson said: "Segregation is an absolute last resort for those deemed a danger to themselves or others.
"Prisoners are entitled to the same care as they would receive in the community, which is why we guarantee the most vulnerable individuals are able to access mental health support tailored to their needs."