Clara White, the sister of Thomas White who has spent nearly 12 years in prison for stealing a mobile phone, said Labour now bore responsibility for fixing sentences of imprisonment for public protection (IPP),which have been denounced as a form of "psychological torture".

Ms White, who has long campaigned for remaining IPP prisoners to be resentenced, told Sky News the new Labour government must "urgently" address the fact that nearly 3,000 people remain in jail under the sentence - despite the fact it was abolished more than a decade ago.

Her intervention comes as a number of campaigners and human rights lawyers wrote to Shabana Mahmood, the new justice secretary, to ask that she work "at pace" to act on the IPP scandal.

In the letter, seen by Sky News, the campaigners expressed "serious concerns about the ongoing scandal of IPP sentences" and the "intolerable position in which successive governments have placed prison and probation staff who manage those still serving these sentences, both in custody and in the community".

"Working at pace to resolve the IPP scandal is the right thing to do," they said.

The campaigners also argued that resentencing and potentially releasing the remaining IPP prisoners could make a "tangible, politically palatable, contribution to addressing the urgent population pressures facing the prison system" - including overcrowding.

What are IPP sentences?

IPP is an open-ended prison term the courts could impose from 2005. They were intended for 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 - who often ended up serving years beyond their initial term.

IPPs were abolished by the coalition government in 2012, but the change was not retrospective - meaning 2,852 prisoners remain behind bars, including 1,227 who have never been released.

The new government is under increasing pressure to act on the IPP crisis given they were introduced by former Labour home secretary Lord Blunkett - who has since said he feels "deep regret" about the way the sentence was used.

Earlier this year Ms White contacted the former home secretary to help her brother, who was was given a two-year minimum jail sentence under an IPP for stealing a mobile phone.

The sentence was handed down four months before the sentences were abolished. Twelve years later, Mr White, now 40, has no idea if he'll ever be let out of prison.

Mr White's family has repeatedly raised the impact that sentence had had on his mental health, which has deteriorated to the extent that he has now been diagnosed with schizophrenia and suffers from hallucinations.

Ms White said in March she feared her brother would take his own life in prison.

Speaking to Sky News on Wednesday, she spoke of how Thomas had recently set fire to his own cell - out of what she called "desperation".

"Coming into power in the way they have, Labour needs to start at the most urgent issues," she said.

"This is urgent and it needs looking at now, straight away - no more time can be wasted.

"They can't wait any longer."

A solution to overcrowding?

In their letter, the campaigners also argue that a resentencing exercise for remaining IPP prisoners, as recommended by the Commons Justice Select Committee, could significantly reduce the overcrowded prison population.

Sky News reported that the government is considering lowering the automatic release point, from the 50% mark in an inmate's sentence to 40% or 43% - something that could result in thousands of additional inmates being let out early.

The proposals, which exclude sexual, violent, and terror-related offenders, are being considered as a means to relieve overcrowding in prisons in England and Wales - where only 700 spaces are said to remain for male offenders.

In the letter to Ms Mahmood, the campaigners and lawyers demand that the government sets up an expert committee to advise on the carrying out a resentencing exercise within 18 months.

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It also asks that the Ministry of Justice (MOJ) publishes its first annual report on IPP, which was due to be published by the end of March 2024.

Alex Chalk, the previous justice secretary who lost his seat at the election, announced a number of reforms to the IPP system, including that those who have been released on licence will no longer have to wait 10 years before they can apply to have it terminated.

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In their letter the campaigners urge the new government to bring these reforms - included in the Victims and Prisoners Act - into force.

"IPP sentences are a problem created by parliament, which can only be solved by parliament, including through new legislation," the letter read.

"We urge the new government to honour its commitment, made in opposition, to "work at pace" to resolve this injustice.

"If this injustice is not resolved decisively, we could see the government challenged in court for their failure to act appropriately."

A MoJ spokesperson said: "The prison system is in crisis and we recognise the significant impact this is having on our whole justice system.

"It is right that IPP sentences were abolished. The Lord Chancellor is committed to working with organisations and campaign groups to ensure the appropriate course of action is taken to support those still serving IPP sentences and will respond to this letter in due course."