Director of Public Prosecutions Alison Saunders is facing calls to resign after she was forced into a U-turn over her decision not to charge Lord Janner for alleged child sex crimes.
An independent review overturned Ms Saunders' original ruling that the former Labour peer should face no action over child abuse allegations because he is suffering from Alzheimer's.
A trial of the facts, where a jury hears the evidence against an individual considered too ill for a full trial, is now expected to be held into the 22 offences allegedly committed in the 1960s, 1970s and 1980s.
Simon Danczuk, the Labour MP for Rochdale who has campaigned for the case to be heard in court, said that Ms Saunders was responsible for a "catalogue of errors".
Mr Danczuk said: "I think she should resign for an number of reasons. S he has made a number of bad judgments and she is just not fit to do the job.
"She made mistakes on a case around FGM (female genital mutilation), serious errors in her attempt to prosecute journalists and now another serious error regarding the Lord Janner case.
"She ignored her advisers, took a long time to make the decision, then announced it when parliament was not sitting although there was significant public interest.
"Most important of all, it was a serious mistake because she failed to understand both the mood of the public and the emotional distress she was causing to the alleged victims.
"We cannot have somebody leading the Crown Prosecution Service (CPS) who is cold and dispassionate towards alleged victims."
Peter Garsden, a solicitor representing nine of the alleged victims in the Lord Janner case, said that his clients wanted to see Ms Saunders go.
"As far as my clients are concerned, to a man they have all said that they think that Alison Saunders should resign," he said.
"Now that Lord Janner is going to be prosecuted they are delighted but they still take the view that Alison Saunders has been proved wrong and that they have been exposed to unnecessarily long suffering while they awaited the outcome of this review."
The 86-year-old peer's family strongly denies claims he used his power as an MP for Leicester to abuse vulnerable young boys at a local children's home.
The case has been listed for a first hearing at Westminster Magistrates' Court on August 7 this year.
A judge will have to decide if Lord Janner is fit to plead. If he is not, a jury will be asked to decide whether he did the acts he is charged with.
The judge will also have to rule on whether the defendant should appear during the trial or can be excused on medical grounds.
A trial of the facts is not considered a trial as such because the defendant cannot put forward a defence.
There is therefore no guilty verdict and the court cannot pass sentence.
All it can do is make a hospital order, a supervision order, or an order for the defendant's absolute discharge.
The CPS said the independent review into Ms Saunders's decision, led by David Perry QC, had agreed it was right to assume Lord Janner would be found unfit to plead.
And it claimed both parties also agreed the most likely outcome of the trial of the facts would be an absolute discharge, which is neither punishment nor conviction.
Specialist abuse lawyer Liz Dux, who represents many of Janner's alleged victims for law firm Slater and Gordon, said her clients were "delighted" by today's announcement.
She said: "This is a vindication of our efforts to challenge the DPP's original decision not to charge Janner, which was clearly not in the interest of justice.
"Our clients have waited long enough for their very serious allegations to be brought before a court.
"They have felt deeply frustrated by the criminal justice system. However, this decision has given them more hope of finally establishing the truth."
Ms Saunders's decision in April not to charge Lord Janner was reviewed after six of the complainants made a request under a scheme introduced two years ago giving victims the right to challenge CPS decisions.
She is the first DPP to have a major prosecuting decision reviewed and overturned through the new policy.
An independent inquiry into child sex abuse being led by Justice Lowell Goddard is also due to hear evidence from the complainants.
Ms Saunders said: "I have always said that in my view this was an extremely difficult and borderline case because of the strong arguments on both sides.
"I have also always emphasised my concern for the complainants in this case.
"I understood their need to be heard, which is why I contacted Justice Goddard to ensure that they could give evidence as part of the public inquiry.
"However, the review has concluded that this forum, albeit a public one, cannot substitute for the adjudication of the courts.
"I accept the outcome of the review and will now be bringing this prosecution to allow for that adjudication to happen."
Attorney General Jeremy Wright QC gave Ms Saunders his backing, saying he is "confident of the independence and the integrity of the Director of Public Prosecutions".
He said: "It is right that we now have a victim's right to review those judgements , which Alison helped to bring in, and which has operated as it was designed to in this case.
"Since Alison Saunders has been DPP, the CPS has prosecuted and convicted more child sex offenders than ever before."
Leicestershire Police said it had been made aware of the decision, which relates to an ongoing criminal investigation it is conducting codenamed Operation Enamel.
A spokesman for the force said: "The Operation Enamel team assisted the victim's right to review process by making submissions and providing a significant body of material in support of the public interest test for prosecution."
Retired judge Sir Richard Henriques is also reviewing how previous allegations made against Lord Janner in 1991, 2002 and 2007 had been handled.
The CPS said today it had received a draft of his report, which is being finalised.
A spokesman said it would confirm the CPS's decisions were "wrong" and the handling of the case by both police and prosecutors had been "unsatisfactory".
Ms Saunders said she would not be standing down.
She told the Press Association: "I don't think in any way it's a resigning matter.
"It shows the process we have implemented works, it shows we will look at victims' right to review and overturn decisions when we decide that's the right thing to do.
"Certainly over the last 18 months there is nothing else there that says I should be resigning, so I won't be."