Failures by police and prosecutors meant three chances were missed to charge Lord Greville Janner over sex abuse claims, an independent report has found.
Director of Public Prosecutions Alison Saunders expressed "sincere regret" at the blunders, but this was rejected by a lawyer for the late peer's alleged victims, who said it was "little consolation".
The report by High Court judge Sir Richard Henriques found that in 1991 prosecutors made the wrong decision not to charge Lord Janner after an "inadequate" police investigation.
In 2002 Leicestershire Police failed to pass claims made by a second alleged victim to prosecutors, and in 2007 again a flawed decision was made not to charge the politician when a third man came forward.
Ms Saunders said: "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.
"It is a matter of sincere regret that on three occasions, opportunities to put the allegations against Lord Janner before a jury were not taken. 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."
The peer died aged 87 in December, days after he was found unfit to stand trial after being charged with a string of sexual offences dating back to the 1960s against nine alleged victims, who were mostly under 16 at the time.
Ms Saunders was at the centre of a controversy last year after originally deciding Lord Janner, who had dementia, should not be charged because of his ill health.
That decision was overturned by an independent review. A special hearing known as a trial of the facts had been scheduled for this year, but last week the criminal case was formally dropped following Lord Janner's death, leaving his alleged victims "devastated".
Solicitor Liz Dux, from Slater and Gordon, which represents eight of Janner's alleged victims, said: "Confirmation by the CPS that mistakes were made in handling of past allegations against Lord Janner comes as no surprise to his alleged victims. The CPS's statement only reinforces our clients' complaints that opportunities to prosecute Janner and bring allegations before a court were repeatedly missed.
"Alison Saunders' expression of 'sincere regret' over failures will be of little consolation unless it is followed by proper accountability. It is vital that those who made the decisions which resulted in Janner escaping justice are called to explain their reasoning."
In his report, Sir Richard found that a Leicestershire Police investigation in 1991 failed to cover basic steps such as checking details of whether a 14-year-old boy, referred to as Complainant 1, had shared hotel rooms with Lord Janner, and that only "extremely limited" inquiries were made at the children's homes where he lived.
The alleged victim even went to a wedding with the peer's family, and it was only two decades later in 2014 that a subsequent police investigation found there was film footage of Complainant 1 at the event.
The report found: "I have concluded that the decision not to charge Janner in 1991 was wrong and that there was enough evidence against Janner, on December 4 1991, to provide a realistic prospect of conviction."
A second alleged victim came forward in April 2000 when police in Leicester were investigating abuse in children's homes.
He made a statement claiming he had been seriously sexually abused by Lord Janner, but this was not passed to the CPS in a file submitted in 2002, and no further action was taken.
In 2007, a reviewing lawyer at the CPS, who had also advised in 1991 and 2002, said problems with the credibility of a third alleged victim, Complainant 3, again meant the peer could not be prosecuted.
There was an eight-month delay between the police file being submitted and the charging decision being made.
Sir Richard said: "In my opinion there was sufficient evidence to provide a realistic prospect of conviction in 2007, and Janner should have been arrested and interviewed and his home searched.
"He should have been charged with offences of indecent assault and buggery with Complainant 1, Complainant 2 and Complainant 3."