Police and prosecutors "appeared reluctant to fully investigate" claims made against Lord Janner, a former Labour MP, according to a report by the Independent Inquiry into Child Sexual Abuse (IICSA).
The peer, who was better known as Greville Janner, was Labour MP for Leicester West between 1970 and 1997 and died in 2015.
Professor Alexis Jay, chairman of the inquiry, said there were "numerous serious allegations" that Janner had sexually abused young boys in the 1970s and 1980s.
At the time of his death Lord Janner of Braunstone was awaiting trial in relation to nine men who had been young boys at the time of the allegations, several of whom were in children’s homes at the time.
Lord Janner (right) with David Cameron in the run up to Holocaust Memorial Day in 2011
© AP Photo / Andrew Winning / Lord Janner with Cameron
But in fact 33 people had made allegations Lord Janner had sexually abused them when they were children.
Prof Jay said there had been a "series of failings" by Leicestershire Police and the Crown Prosecution Service.
His report found the police "shut down" investigations into Lord Janner without having fully investigated all the claims.
Janner and his family have always insisted he was innocent and his son Daniel has claimed he was the victim of a witch-hunt.
The IICSA report stops short of saying Janner was guilty of any crimes but says: "On multiple occasions police put too little emphasis on looking for supporting evidence and shut down investigations without pursuing all outstanding inquiries.”
"This inquiry has brought up themes we are now extremely familiar with, such as deference to powerful individuals, the barriers to reporting faced by children and the need for institutions to have clear policies and procedures setting out how to respond to allegations of child sexual abuse," the report added.
The report criticised Leicestershire Council, who were responsible for the children’s homes and also lambasted the Labour Party after hearing that several members of the party in Leicestershire had tried to raise the matter with the national headquarters.
“It was not enough for the Labour Party to have left this matter to the police and the Crown Prosecution Service. Lord Janner remained in a privileged and powerful position as an MP. This position had allowed him access to children’s homes and schools in the past, and may have done so again,” said the report.
The Labour Party has since introduced a safeguarding system which would prevent a repetition of what happened with Lord Janner.
Daniel Janner said the inquiry had offered “no proof whatsoever of guilt."