An independent QC is to overturn the controversial decision that Lord Janner should not face justice over a string of paedophile allegations.

The sensational U-turn by prosecutors means a court will now hear historical child sex claims against the disgraced former Labour MP.

A top barrister, who has spent several weeks examining the decision not to put Janner on trial, has concluded the allegations should be heard even though the peer has severe dementia.

U Turn: An independent QC is expected to overturn the controversial decision not to prosecute Lord Janner, who is accused of 22 sex attack over 19 years but now suffers from advanced dementia

Confused: Lord Janner pictured outside his house in London in 2014. It is said he now needs round the clock care as he suffers from advanced dementia

The QC is said to have serious reservations about the original decision not to charge the 86-year-old and is to recommend to Director of Public Prosecutions Alison Saunders that it be overturned. Barring a last minute change of mind, his ruling – due to be made public next week – paves the way for the evidence against Janner to be tested in a criminal court – in a procedure known as a ‘trial of the facts’ – even if he is too unwell to attend.

Never before has such a high-profile Crown Prosecution Service decision been overturned following a review.

The development will pile pressure on Mrs Saunders, who has suffered a torrid two months amid questions over her handling of the Janner affair. She is expected to face renewed calls from critics to consider her position.

The extraordinary twist in the case comes after a group of Janner’s alleged victims applied for a formal review of the decision not to charge him.

The appeal was launched after Mrs Saunders ruled the peer should not be charged on health grounds, despite saying there was enough evidence to prosecute him for 22 sex offences against nine people. She also ruled out holding a trial of facts, which can be used when suspects are unable to enter pleas or instruct lawyers.

Alison Saunders said that Lord Janner, who suffers from advances dementia, should not face trial

Janner, who has advanced dementia, was accused of carrying out a catalogue of abuse against young boys.

More than a dozen people came forward to claim he abused them during the 1960s, 1970s and 1980s.

His family has repeatedly denied he is connected to any wrongdoing.

Following Mrs Saunders’ decision to rule out a prosecution, a cross-party group of MPs said her ruling was damaging public confidence. Campaigners claimed there had been an Establishment cover-up – an allegation emphatically denied by Mrs Saunders, who said she prided herself on her independence.

Responding to criticism, the DPP insisted she had made the right decision not to prosecute Janner, adding that his dementia was so severe he could ‘play no part in a trial’.

Her supporters say that the independent review is evidence that the Crown Prosecution Service is ‘transparent and listens to victims’, although a U-turn in so prominent a case could call into question the judgment of the country’s most senior prosecutor.

Labour MP Simon Danczuk, who has campaigned on behalf of historic abuse victims, said: ‘It looks as though Janner’s case may now be examined in court. This is all the alleged victims have wanted. All we’ve ever asked for is the law to run its course. No one can be above the law.’

Last night Paul Miller, who claims he was groped by Janner at the Palace of Westminster on a school trip when he was nine, said: ‘I was losing hope that this would ever happen. It is great news and I will certainly get a lot of satisfaction. Janner should have been in the dock years ago.’ The 52-year-old added that the development raised the question of ‘what should happen to Alison Saunders now’.

Baron Greville Janner in November 1972 with essay winners at Beaumont Leys Secondary School. Critics said that the decision not to prosecute him was an establishment cover up

Another alleged victim, Hamish Baillie, 47, said: ‘Yes, Janner will now have to face a court. But this doesn’t change the fact that for decades [his] abuse was ignored and covered up. The corruption is still there.’

It is unclear why the reviewing QC is overturning the decision. It is understood that as part of his inquiry, the barrister would have looked at previous cases involving frail, mentally ill defendants.

Due to Janner’s ailing mental capacity, police will not be able to charge him. It is thought more likely he will be summonsed to appear in court. It is unclear whether he will be well enough to turn up and his lawyers are likely to argue the case should be thrown out, because he is too ill to give them instructions – let alone understand the charges.

Janner was diagnosed with Alzheimer’s disease in 2009 and, it is said, requires round-the-clock care.

To be heard: Due to Janner’s ailing mental capacity, police will not be able to charge him. It is thought more likely he will be summonsed to appear in court but the case will at least be heard

THE ACCUSATIONS: 22 SEX ATTACKS SPANNING NEARLY TWO DECADES By Alison Saunders’ own admission, the evidence against Lord Janner was strong enough in relation to no fewer than nine victims to place before a court. The Labour peer is suspected of carrying out 22 sex attacks against young victims, including a girl, who were in local authority care. They allegedly took place over a 19 years and include: 14 indecent assaults on a male under 16 between 1969 and 1988 Two indecent assaults between 1984 and 1988 Four counts of serious sexual assault on a male under 16 between 1972 and 1987 Two counts of serious sexual assault between 1977 and 1988 25 victims are thought to have contacted police Around 25 victims are thought to have contacted police as part of the investigation. The main claims investigated by Leicestershire Police centre on Janner’s alleged friendship with Frank Beck, a paedophile care home manager jailed in 1991 for a string of sex offences. One of those prepared to give evidence against Janner is Hamish Baillie, 47, who lived in Beck’s care home and alleges that at 15 he was molested by the peer during a game of hide-and-seek. Another complainant who formed part of the inquiry is Ray Dunkley, 56, who said he was indecently assaulted when Janner visited his primary school in 1966 – four years before he was elected an MP. Since the decision in April not to prosecute Janner, more alleged victims are believed to have come forward, with at least 30 now said to have spoken to police. Police have launched a separate investigation into allegations that Janner took a teenage boy to Scotland in the 1970s and sexually assaulted him. Advertisement

But since Mrs Saunders’ original decision, there have been a series of revelations about his health. Last week it was reported Janner made secret official visits to Parliament months after police were told he was too ill to be questioned.

Although the former Labour MP for Leicester West has not been interviewed by police, his home in Hampstead, north west London, was raided in December 2013, and his office in Westminster was searched in March 2014. The child sex allegations against Janner surfaced at the trial of paedophile care home boss Frank Beck in 1991. Police investigated the politician again in 2002 and 2007. There are now four separate reviews into the repeated failures to put him on trial.

Asked about the review last night, a CPS spokesman said: ‘No decision has been made yet. The review has not concluded. We don’t have any confirmed timings as yet on when it will be concluded.’

Mrs Saunders received a pay packet of almost £600,000 last year, The Daily Telegraph

It is understood that as part of his inquiry, the barrister would have looked at previous cases involving frail, mentally ill defendants

Trial of facts but no way he can be punished: JAMES SLACK'S analysis

The case against Lord Janner is likely to be tested under a little-known law which allows for a ‘trial of the facts’ – but there is no prospect of him receiving any punishment.

Under the Criminal Procedure (Insanity) Act 1964 a judge can decide, on medical evidence, that a person is unfit to plead.

In the case of Janner, he has been assessed by four medical experts – two acting for the Crown Prosecution Service and two for the Labour peer’s defence team. All agreed that, owing to degenerative dementia, his evidence could not be relied upon in court and he could not have any meaningful engagement with the court process.

Life peer British barrister, writer, journalist and broadcaster and former Labour MP In robes for the introduction of Baron Bach into the House of Lords

The judge in charge of the case will, at the outset, make a formal declaration in open court that he is unfit for trial. He will then ask a jury to decide – on the basis of evidence given by prosecution lawyers, including statements provided by alleged victims – whether or not Janner did the acts he was accused of.

A defence team will be able to put the case for Janner, though he will not need to be in court.

The law is clear that it is not a formal trial, and should not be described as one, because the defendant cannot put forward a defence himself.

As a result, there is no verdict of guilty and the court cannot pass a criminal sentence. All it can do is to make a hospital order, a supervision order or an order for the defendant’s absolute discharge. Essentially, it is there to protect the public – not deliver a punishment. In Janner’s case, because of his age and ill health, whatever the outcome he is highly unlikely to be considered any threat.

The decision to seek a ‘trial of the facts’ will be seen as a humiliating rebuke for Director of Public Prosecutions Alison Saunders – who considered the idea and dismissed it.

In her statement announcing Janner would face no action, she said there was no public interest in pursuing a course that would inevitably end in an absolute discharge. What she overlooked – at great cost to her own reputation – was the determination of the alleged victims to have their evidence put before a court.

Recent examples of findings of fact hearings include the case of ex-Labour MP Margaret Moran, who falsely claimed £53,000 in expenses.

The jury heard the case in her absence after a judge ruled she was unfit to stand trial for mental health reasons. Moran was placed under a supervision and treatment order.