Multimillion-pound Government plans to keep serious criminals behind bars for longer could prompt re-offending, radicalisation and violence in jails, official reports have warned.

The impact assessments carried out for the Sentencing White Paper – which Justice Secretary Robert Buckland said would result in a “fairer system” that better protects the public – said prisons were at risk of instability and overcrowding with the prospect of inmates becoming more dangerous if the plans were implemented.

The £500 million shake-up of court powers promises longer jail terms for serious offenders, requiring them to serve a third of their term behind bars before they can be assessed for release as opposed to half.

This could lead to around 2,400 prisoners serving longer jail terms by 2028.

The best estimate net cost of the changes is £542.6 million over 10 years, according to an impact assessment for the White Paper.

The Government has also pledged community punishments could include 20-hour daily curfews for up to two years and GPS tracking for burglars, robbers and thieves when they are released from prison.

During a speech for the Centre for Social Justice on Wednesday, Mr Buckland said the white paper was an “opportunity to grow trust and confidence in the sentencing system – in its ability to make the smart choices to protect the public from the harmful effects of crime, in whatever form they take”.

The announcement comes days after a parliamentary report warned prisons could run out of space to hold more criminals within the next three years, with some higher-risk inmates already having to be held in low-security jails.

Meanwhile courts in England and Wales are grappling with a backlog of more than half a million cases, with delays exacerbated by the coronavirus pandemic.

At the same time, temporary laws are also being introduced to extend the custody time limit for people awaiting a trial for serious crimes.

One of the impact assessments said the prison and youth custody services face “increased population and longer times spent in custody for some offenders which may compound prison instability, self-harm, violence and overcrowding.

“Serving longer periods in custody may mean family breakdown is more likely, affecting prisoner mental health and subsequent re-offending risk.”

There may be an “increased risk that other prisoners could become radicalised or more dangerous due to the greater time in custody for the affected individual affording more time for further proselytising in the prison population,” it added.

Swelling inmate numbers could affect the ratio of staff to offenders and the ability to provide activities and time spent out of a cell, “a factor associated with increased levels of violence”, the report said.

Another document warned of the potential risk to “prison stability” which could lead to “increased tensions in prison establishments, with consequent impacts on prisoner violence or self-harm”.

The plans also include:

– Whole life orders for child killers, including for 18-20 year-olds in exceptional cases.

– Measures to rehabilitate low-level criminals to reduce re-offending, like cutting the time before a conviction is classed as “spent” so they can find work and have flexible curfew rules around a job.

– Efforts to encourage deferred sentences, where criminals are given a chance to turn their life around before a judge imposes a punishment, as alternatives to custody in certain circumstances.

– A pilot for five “problem-solving” courts to tackle low-level offending.

– Addressing the approach to offenders with conditions like autism and dyslexia.

Campbell Robb, chief executive of social justice charity Nacro – which supports prisoners, said “senselessly banging people up for longer” would not create a criminal justice system “fit for the 21st century” and this would “only add more pressure to this already stretched system”.

While James Mulholland QC, chairman of the Criminal Bar Association, said a sentencing review “matters little when police-reported serious crime continues to rise; yet charging rates and prosecutions plummet and the tiny fraction of offences that do make it to court are left up to five years without being completed.

“A Government that concentrates on sentencing the back-end of justice, whilst failing to pay up and invest significantly more and swiftly into policing, CPS and courts, fails in its core duty to keep the public from harm.”