Lord Grenville Janner could have been prosecuted for alleged child sex abuse on three occasions over the past 25 years, an inquiry has found.
The findings come days after criminal proceedings against the politician were dropped by Mr Justice Openshaw because of his death in December.
The ruling at the Old Bailey allows the findings of an independent inquiry into the handling of past allegations into past allegations to be published for the first time.
Sir Richard Henriques, a retired High Court judge, found three occasions where police and the Crown Prosecution Service (CPS) should have pursued the former Labour peer and MP.
The first was in 1991, when a former Leicestershire children's home director being tried for child abuse accused Lord Janner of the same offence.
The CPS advised police that there was insufficient evidence to prosecute, despite a witness in the case claiming he was abused by the politician at the age of 13.
“The decision not to charge Lord Janner in 1991 was wrong and there was enough evidence against him to provide a realistic prospect of conviction for offences of indecent assault and buggery,” Sir Richard’s report said.
“In addition, the police investigation was inadequate and no charging decision should have been taken by the CPS until the police had undertaken further enquiries.”
The second missed opportunity came in 2002, when allegations referred to the CPS as part of Operation Magnolia did not include those against Lord Janner, making further action impossible.
Four years later, the Leicester West MP was named once again alongside two other individuals, being accused of indecent assault
The CPS once again decided there was insufficient evidence to prosecute Lord Janner for indecent assault and “buggery” but the report found he should have been arrested, questioned and had his home searched.
In his report, Sir Richard said he had “no doubt” that case management panels acting under the CPS’s current procedures would have taken all three cases forward.
Legal guidance on child sexual abuse introduced in 2013 stipulates that specially-trained prosecutors should handle rape and serious sexual offences and not let the credibility of the allegation but undermined by late reporting and other factors that previously prevented charges.
Sir Richard made recommendations to the CPS asking it to consider whether time limits in charging decisions are appropriate, to refer to caseworkers before closing cases and to establish a “central log” of referrals, whether charges are brought or not.
Lord Janner denied the allegations against him and his family said he was “entirely innocent of any wrongdoing”.
Alison Saunders, the Director of Public Prosecutions, commissioned the inquiry last year after facing a storm of criticism over her original decision not to charge the peer because he suffered from dementia.
She found there was sufficient evidence to charge Lord Janner with 22 counts of child sex abuse between 1969 and 1988 but that it was “not in the public interest” to put him on trial.
Ms Saunders said the missed opportunities were a “matter of sincere regret”.
“The inquiry's findings that mistakes were made confirms my view that failings in the past by prosecutors and police meant that proceedings were not brought,” she added.
“It is important that we understand the steps which led to these decisions not to prosecute, and ensure that no such mistakes can be made again.”
She said Sir Richard’s conclusions had been carefully considered and that although the CPS had moved on “hugely” and charges would have been pressed under current procedures, the recommendations would be acted on.
All case files and papers have now been handed to the Goddard Inquiry, which is investigating historical sexual abuse in England and Wales.