The number of prisoners being jailed for life has nearly doubled in the past 10 years and sentences served are now more than 50% longer than they were when first introduced, despite claims that judges are being too lenient.

"Courts have got much tougher, handing down longer sentences and making greater use of custody than ever before," said Juliet Lyon, director of the Prison Reform Trust. "In the heat of the current debate, it is easy to miss that clarity."

According to the latest Home Office statistics, contained on their website, "the largest proportionate increases since April 2005 were for those sentenced to indeterminate sentences (life sentences and indeterminate sentences for public protection), which increased by 20%." The statistics also show nearly 7,000 prisoners are serving such sentences.

Last year there were 6,431 prisoners serving life sentences, a rise of 12% on the previous year. There were fewer than 3,000 "lifers" in 1992. The latest annual figures show 570 people were jailed for life last year, compared to 252 10 years earlier. In 1965, "lifers" served an average of nine years, which had increased to 10.3 years by 1980 and to around 15 years today.

Part of the problem is that no distinction is made in the figures between the prisoners who are serving time for murder and manslaughter and those who are in for lesser offences. "Successive governments have brought in a raft of legislation and at the same time failed to explain sentencing policy to the public," said Ms Lyon.

John Hirst was jailed for life for the manslaughter - on the grounds of diminished responsibility because of mental problems - of his landlady in 1980, with a tariff of 15 years, later increased by the home secretary to 18 years. He was released in 2004 after serving 25 years.

"In those days, they didn't tell you what your tariff was," he said yesterday. "I thought it (the average tariff) was seven or eight years at the time and it has since doubled." He said that all fellow "lifers" he knew of, apart from one, had served more than their tariffs. "The only others who were released were on death's door."

He accepts that the relatives of the woman he killed might have wanted him to spend the rest of his life inside. "The daughter (of his landlady) was in the public gallery at the trial. Until then, I hadn't thought about her at all. She hated me, I could tell from the way she looked at me. She was very angry and I understand that. For the first time, I realised that there was another victim. I had learned my lesson before I was even sentenced."

Organisations representing the victims of murder and manslaughter have also called for clarification. They are angry that prisoners jailed for murder are freed when judges have said they would never be.

Rose Dixon of Support after Murder and Manslaughter (Samm) said that one case involved a man who had killed a woman and badly beaten her sister and who had been told by the judge that life would mean life. "He was released 11 or 12 years later," she said. "Families feel as though nobody listens to them."

Former prison governor Tim Newell, has dealt with many lifers. His last prison, Grendon Underwood, treated prisoners convicted of serious violent and/or sexual offences. He cited instances of young men leaving prison after serving a relatively short sentence, but who will remain on licence - and under supervision - for the rest of their lives. He said that the introduction of life sentences not related to murder or manslaughter had changed the dynamics of the justice system and that the probation services cannot cope. Now a consultant with the Butler Trust, a charity promoting effective care for offenders, Newell said the media emphasis on victims supporting longer sentences is misleading. "When you talk to them - victims - they seek understanding of what has happened and they are often pleased to hear about the positive programmes in prisons which are aimed at making sure that what they have gone through doesn't happen to anybody else."

Around the world

• Japan: Age of consent for sexual activity is 13, but under the penal code penalty for rape of a minor is the same as for rape of an adult. Punishment for both offences is a minimum of two years' hard labour. If the victim dies or is injured during the rape, minimum sentence is three years' hard labour. Rape, whether of a minor or an adult, is only prosecuted on complaint.
• Denmark: Anyone who has sexual intercourse with a child under 15 is liable to a maximum jail term of eight years. If the child is under 12 or the offender uses coercion, maximum penalty is 12 years. There are provisions for Danish citizens who commit these offences overseas to be tried in Denmark.
• Albania: Penal code prohibits sexual intercourse with girls under 14 or girls who have not reached puberty. Punishment is a minimum jail term of five years and a maximum of 15 years. If the intercourse involves serious physical harm, minimum sentence is 10 years and maximum is 20. If victim is killed, or later commits suicide, minimum sentence is 20 years.
• Thailand: Thai penal code outlaws sexual intercourse with a girl under 13, with or without consent. Punishment is minimum of seven years and maximum of 20 and a fine of 14,000 to 40,000 baht, but a life sentence can be imposed. If the rape is committed with an intent to murder, or the rapist uses "any gun or explosive", the offender is imprisoned for life.
• Saudi Arabia: Saudi Arabia is governed by Islamic law (Sharia), which outlaws all sexual intercourse outside marriage, irrespective of age, but the person is discharged from responsibility if she is a minor and did not give consent. Maximum penalty for rape is execution by beheading or stoning