Thousands of criminals could have their jail terms cut after one of Britain’s most senior judges said that courts were justified in giving lower sentences where prisoners faced overcrowded jails.

Sir Igor Judge, who is the first Head of Criminal Justice, believes that judges should reduce the “punitive” element of a sentence if prisoners are going to be locked up in “dreadful conditions”.

Judges, he believes, are justified in cutting jail terms if two prisoners would be confined to a cell designed for one or if they were denied exercise facilities. That, in his view, is the “correct approach to sentencing arising from prison overcrowding”.

His comments come as the prison population reaches record levels. On Friday it was 81,474, including 339 in police cells. In 2006-07 nearly 18,000 prisoners were serving their sentences doubled up in cells designed for one.

The views of such a senior judge will not be ignored by the Sentencing Guideline Council because he is its deputy chairman. One senior legal expert told The Times that the comments could now lead to new guidelines being issued.

If taken to heart by judges, the new approach will help to ease the crisis of prison overcrowding by cutting the jail terms of thousands of offenders. But ministers would be acutely embarrassed by offenders receiving lesser sentences just because of poor prison conditions.

Nick Herbert, the Shadow Justice Secretary, said last night: “Sentencing should fit the crime, not the prison capacity. It is quite wrong that the Government’s failure to plan for sufficient prison space is resulting in judicial discretion being fettered. This makes another mockery of Gordon Brown’s promise to want to punish criminals.”

Sir Igor, also President of the Queen’s Bench Division for England and Wales, told a meeting of the Prisoners’ Education Trust: “I have believed for some time that you have to take into acount, in the punitive element of the sentence, that in conditions that are wildly overcrowded, you may be serving your sentence in dreadful conditions, locked up with one or two other people or forced into a sitution where there is no exercise.”

Because the punitive element of the sentence was therefore that much harsher, he said judges were “perfectly justified in saying: if you are going in for nine months, it can be reduced”.

The move would not mean dangerous or violent prisoners being let out while still posing a risk to the public. Although the “punitive” part of their jail term might be shorter — the mininum time to be served for “retribution and deterrence” — they would still be kept inside until judged safe for release.

His comments became public as the Government indicated yesterday that because of jail overcrowding it could recommend that offenders who would normally be given short custodial terms would go to prison only if there was sufficient space.

David Hanson, the Prisons Minister, told the cross-party Justice Unions Parliamentary Group that he was “actively considering” the plan.

However, in his lecture last week Sir Igor said he believed that it was unacceptable for judges to avoid custody altogether simply because there was no room. “That will bring the whole system into disrepute,” he said.

A series of Court of Appeal rulings has made clear that judges can take account of resources in the prison system when deciding a sentence in cases where they have a choice of penalty and custody is not needed for the protection of the public.

But there has been a furore whenever ministers themselves have stepped in and commented on the need for judges to take account of prison spaces. In January 2005 Charles Clarke, then Home Secretary, proposed but later abandoned the idea that jail sentences be linked to whether there were enough prison places — the first time in British history the suggestion had been made by a minister.

He had planned that the Sentencing Guidelines Council (SGC), a panel of judges, lawyers and academics set up to gain more public input on sentencing policy, would consider the cost of different sentencing and effectiveness in preventing repeat offences.

That was followed in January this year when ministers drew the attention of the National Criminal Justice Board to the pressures on prison places, and the Lord Chief Justice, Lord Phillips of Worth Matravers, told judges and magistrates that there was “tried and tested” authority to take acount of overcrowding when considering penalties.

An angry reaction followed from some judges, who said that ministers were interfering with sentencing and Lord Phillips, who is chairman of the SCG, was forced to issue a second statement explaining that ministers were not telling judges what to do.

A Ministry of Justice spokeswoman insisted that measures were in hand to increase jail capacity.

“We will always make sure there are prison places for those serious and dangerous offenders who ought to be in prison, and that courts have tough community sentences at their disposal to deal with less serious, non violent offenders,” she said. “The National Offender Management Service is building more capacity in the prison estate as part of the response to population pressures and continues to investigate options for providing further increases in capacity.”

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