Leicestershire's police and crime commissioner (PCC) has described as "perverse" the decision not to charge Lord Janner for alleged sexual abuse of children.
The Crown Prosecution Service (CPS) said the Labour peer and former Leicester MP would not face charges because of the severity of his dementia.
The move was described as "perverse" by Leicestershire's PCC Sir Clive Loader.
The abuse allegations relate to residents in children's homes in Leicestershire in the 1970s and 1980s.
More than a dozen individuals made allegations to police relating to Lord Janner, 86, a former president of the Board of Deputies of British Jews.
The "core allegation" was that as Labour MP for Leicester West at the time, Greville Janner befriended the manager of a children's home to allow him access so that he could "perpetrate serious sexual offences on children".
Leicestershire Police interviewed more than 2,000 people throughout the course of its most recent investigation launched in 2013, and a "comprehensive file of evidence" was submitted to the CPS.
A really bad decision
Sir Clive, a Conservative, said he was "really angry" at the decision not to charge Lord Janner, describing it as "contrary to any notion of natural justice".
"I cannot believe that any right-minded person will understand or support it," he said.
"For decades, this man is alleged to have carried out premeditated, systematic sex crimes against young boys and one girl who were in the care of the local authority.
"I think this is a really bad decision. It's really bad for these victims; it's bad for other victims of a similar crime, who perhaps won't now come forward."
He said there was no dispute that Lord Janner was not in a fit state to act in his own defence, but that another mechanism - a hearing of facts - could be held.
"I don't know why it has been decided this couldn't happen, but I think it is a really bad decision."
Lord Janner was diagnosed with Alzheimer's disease in 2009.
His family said: "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.
"As the Crown Prosecution Service indicated today, this decision does not mean or imply that any of the allegations that have been made are established or that Lord Janner is guilty of any offence."
Director of Public Prosecutions Alison Saunders said there was enough evidence to charge him with 22 sex offences, but he was now too sick to stand trial.
Ms Saunders admitted the case was not "thoroughly investigated" at the time and only properly looked at under the most recent investigation, Operation Enamel.
She said: "It is a matter of deep regret that the decisions in relation to the previous investigations were as they were."