The supreme court will hear a challenge by four offenders on Monday who allege that indeterminate sentences infringe the rights of prisoners if they are unable to get on to rehabilitative courses.

Britain has more prisoners serving life sentences than the rest of Europe put together, with more than 12,000 inmates on either life terms or indeterminate sentences.

The average length of time served by lifers and those given indeterminate prison terms is increasing rapidly, the latest Ministry of Justice (MoJ) figures reveal.

The rise comes amid mounting criticism from MPs and civil rights groups that inmates are finding it harder to enter rehabilitation courses that would pave the way to their release and ease prison overcrowding.

The MoJ's figures show average time served by those on mandatory life sentences has risen from 14 years in 2006 to 17 years in 2013. For other lifers, the average span doubled over the same period from seven years in 2006 to 14 years in 2013.

The average time spent in prison by those serving indeterminate terms for public protection has also gone up, from one year in 2006 – when the sentence came into effect – to six years in 2013.

Increase in sentence lengths is one of the main drivers of the expansion of the adult prison population. In a report last month, Managing the Prison Estate, the Commons public accounts committee said prison overcrowding could be reduced and savings made "if [the MoJ] provided more offender behaviour programmes to help prisoners serving indeterminate sentences to be released at the earliest opportunity".

In June 2013, the report noted, the prison population included thousands on indeterminate sentences who had already served the minimum period required by their sentence. It added: "They could be released if the Parole Board believed it was safe to do so. The Parole Board views offender behaviour programmes as very important to demonstrate progress, but the number of courses completed by prisoners has fallen."

The Howard League for Penal Reform and the Prisoners' Advice Service fear that the MoJ's decision to remove legal aid for inmates challenging the types of prisons in which they are being held may reinforce difficulties prisoners face in transferring to open prison and beginning the process of rehabilitation.

The proportion of inmates held on longer sentences has been rising steadily. By the end of last year there were, according to MoJ statistics, 7,463 people who had been given a life term and 5,335 on indeterminate sentences for public protection, a total of 12,798 inmates.

Frances Crook, chief executive of the Howard League for Penal Reform, said: "We have sentence inflation. It's because politicians in the last two decades have sunk to the level of punitive competitiveness. But there's no evidence that it offers better public protection.

"We have more lifers than all the other countries in the Council of Europe together. This government came in with good intentions but there are still so many prisoners sitting around [awaiting rehabilitation courses]."

Those sentenced to life or indeterminate sentences "have no automatic right to be released", the MoJ stresses. "Release on expiry of the tariff period is not automatic. Release will only take place once [the tariff period] has been served and the Parole Board is satisfied that the risk of harm the prisoner poses to the public is acceptable."

Life sentences can be imposed for a wide range of offences. The maximum term for rape, arson, manslaughter, torture, hijacking, supplying class A drugs, robbery, aggravated burglary, carrying firearms, causing explosions and procuring a miscarriage is life imprisonment. A life sentence is mandatory for anyone aged over 21 who is convicted of murder.

Even among lifers there are multiple categories, reflecting a proliferation of sentencing regimes that have changed over time. There are prisoners serving automatic life sentences, discretionary life sentences, those held on detention during Her Majesty's pleasure, in custody for life – and other permutations.

The most notorious are the 55 inmates – such as Mark Bridger, who was sentenced for the murder of five-year-old April Jones, and Michael Adebolajo, the killer of Fusilier Lee Rigby – who are serving whole-life terms. They will never be released from jail.

Their status has caused friction with the European court of human rights in Strasbourg, which ruled that such sentences must be subject to review and therefore hold out the possibility of release. As they stand, the ECHR said, whole-life terms are inhuman and degrading – a judicial rebuff that the court of appeal has since, in effect, sidestepped.

The MoJ counts those on indeterminate sentences, who must satisfy the Parole Board before they can be freed, with lifers. That agglomeration of categories explains why research by the Howard League in 2009 first came up with the astonishing statistic that there are more people serving open-ended or life sentences in jails in England and Wales than in the whole of the rest of Europe.

The latest statistical report from the Council of Europe, assessing prison populations in mid-2012, confirms that disparity. There were more than 8,800 serving life terms in Britain compared with about 7,000 in the rest of Europe.

Other European states are, by comparison, reluctant to impose life sentences and generally reserve the term "life" for whole-life sentences. Several – Spain, Portugal, Norway, Croatia and Serbia – have even abolished what they call life terms.

Inmates aged more than 60 are the fastest growing section of the prison population. In September 2012, there were 3,333 older inmates. By December 2013, the figure had risen to 3,536 – of whom 99 were women.