Greville Janner was Labour MP for Leicester North West and then Leicester West Labour peer Lord Janner will not face child sex abuse charges because the severity of his dementia makes him unfit to stand trial, the Crown Prosecution Service (CPS) has said.
The abuse allegations relate to residents in Leicestershire children's homes between 1970 and the 1980s.
Police condemned the CPS decision as "wrong", and the Labour Party has suspended the 86-year-old peer.
In 1991 Lord Janner told Parliament he was innocent of abuse allegations.
A High Court judge will now review the CPS's previous handing of the case.
More than a dozen individuals made allegations to police relating to Lord Janner, the CPS said.
Leicestershire Police has been investigating abuse allegations against him and said it was "worried" about the impact of the decision.
Explaining its decision, Director of Public Prosecutions Alison Saunders said there is no treatment for Lord Janner's condition and there is no current or future risk of offending.
His condition is "rapidly becoming more severe" and requires continuous care both day and night, her statement said.
"His evidence could not be relied upon in court and he could not have any meaningful engagement with the court process, and the court would find it impossible to proceed."
Ms Saunders said the decision will be "extremely disappointing to complainants". She said she has written to all the complainants.
The Assistant Chief Constable Roger Bannister said he believed the decision was "the wrong one" and it would do little to support and encourage victims of sexual abuse to come forward.
He said: I am extremely worried about the impact the decision not to prosecute him will have on those people [who came forward].
"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."
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