A Labour peer accused of "some of the most serious sexual crimes imaginable" will not face trial because of the "severity" of his dementia, the Crown Prosecution Service (CPS) has confirmed.

Lord Greville Janner would have been charged with a string of historic sex offences against children, the CPS said on Thursday.

The 86-year-old would have been charged with 14 indecent assaults on a male under 16 between 1969 and 1988; two indecent assaults between 1984 and 1988; four counts of buggery of a male under 16 between 1972 and 1987; and two counts of buggery between 1977 and 1988.

More than a dozen people came forward to claim they were abused by Lord Janner, who is alleged to have used his influence as a Labour MP for Leicester to prey on vulnerable young boys at local children's homes.

He was diagnosed with Alzheimer's disease in 2009 and has been on a formal leave of absence since October 2014.

In a statement, the CPS said it "considers that the evidential test was passed on the basis that the evidence is sufficient to have warranted charging and prosecuting Lord Janner in relation to the particular charges" relating to nine of the individuals.

However, the statement by the Director of Public Prosecutions Alison Saunders concluded that the severity of his dementia means he is not fit to take part in any proceedings. “There is no treatment for his condition, and there is no current or future risk of offending,” her statement added.

Leicestershire Police said it was disappointed at the decision not to prosecute Lord Janner, condemning it as the "wrong one". Assistant Chief Constable Roger Bannister confirmed the force is exploring any possible legal avenues to challenge it.

In pictures: White Flowers Campaign to support child abuse victims Show all 10 1 /10 In pictures: White Flowers Campaign to support child abuse victims In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign A campaigner holds a banner at Old Palace Yard in Westminster, during an event organised by the White Flowers Campaign Group, in commemoration of victims and survivors of child abuse In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign Pictures, messages and floral tributes left at Old Palace Yard in Westminster, organised by the WhiteFlowers Campaign Group in commemoration of victims and survivors of child abuse In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign Jenny Tomlin, the mother of Martine McCutcheon, attends a gathering at Old Palace Yard in Westminster, organised by the White Flowers Campaign Group, to lay white flowers in commemoration of victims and survivors of child abuse In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign Child abuse campaigner Bill Maloney speaks to relatives, supporters, victims and survivors of abuse as they gather at Old Palace Yard in Westminster to lay white flowers in commemoration of victims and survivors of child abuse In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign Rotherham MP Sarah Champion speaks during a gathering at Old Palace Yard in Westminster, organised by the WhiteFlowers Campaign Group, to lay white flowers in commemoration of victims and survivors of child abuse In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign Simon Danczuk MP speaks during a gathering at Old Palace Yard in Westminster, organised by the WhiteFlowers Campaign Group, to lay white flowers in commemoration of victims and survivors of child abuse In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign John Mann MP attends a gathering at Old Palace Yard in Westminster, organised by the WhiteFlowers Campaign Group, to lay white flowers in commemoration of victims and survivors of child abuse In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign A woman leaves a floral tribute at Old Palace Yard in Westminster In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign Relatives, supporters and survivors of abuse hold a minutes silence during a gathering at Old Palace Yard in Westminster, organised by the WhiteFlowers Campaign Group, in commemoration of victims and survivors of child abuse In pictures: White Flowers Campaign to support child abuse victims White Flowers Campaign Jenny Tomlin, the mother of Martine McCutcheon (left) supports Esther Grace of Cardiff, a victim and survivor of abuse, during a gathering at Old Palace Yard in Westminster, organised by the WhiteFlowers Campaign Group, to lay white flowers in commemoration of victims and survivors of child abuse

ACC Bannister said: “There is credible evidence that this man carried out some of the most serious sexual crimes imaginable over three decades against children who were highly vulnerable and the majority of whom were in care.

“I am extremely worried about the impact the decision not to prosecute him will have on those people, and more widely I am worried about the message this decision sends out to others , both past and present, who have suffered and are suffering sexual abuse.

“We are exploring what possible legal avenues there may be to challenge this decision and victims themselves have a right to review under a CPS procedure.”

His family issued a statement through his lawyers saying: "Lord Janner is a man of great integrity and high repute with a long and unblemished record of public service.

"He is entirely innocent of any wrongdoing."

The CPS conceded that Lord Janner should have been prosecuted over these complaints, saying "mistakes were made" by both the Leicestershire police in 2002 and the CPS in 1991 and 2007.

"It is a matter of deep regret that the decisions in relation to the previous investigations were as they were," the statement said.

"Had the previous decisions been to prosecute, as they should have been, Lord Janner would have had the opportunity to challenge the evidence and defend himself through the trial process, with a jury ultimately deciding on his guilt or innocence some years ago."

It said the victims of the alleged offences had been denied the chance to pursue criminal proceedings, adding: "It is of obvious and particular concern that such proceedings did not take place as a result of what the CPS now consider to be wrong decisions".

The retired High Court Judge, Sir Richard Henriques, has been asked to conduct an independent review into the CPS' decision making and handling of the case in order to "maintain public confidence in the administration of justice and to seek to learn appropriate lessons".

Lord Janner served as an MP for 27 years, first for Leicester North West and then Leicester West. He was made a life peer in 1997 when he retired.

His House of Lords office was searched by police in March last year, but he was not arrested. In 2013 his home in Barnet, north London was raided by police. His name was also mentioned in the trial of Frank Beck, who was convicted for sex offences. He strongly denied any wrong-doing, telling Parliament: "There was, of course, not a shred of truth in any of the allegations of criminal conduct made against me during the trial."