Lord Janner has been ordered to appear in court to face allegations he abused children in the 1960s, 1970s and 1980s.
Westminster Magistrates Court ruled he was legally required to attend court to face the hearing, even if he was unable to understand or play a part in proceedings.
However his lawyers claimed he was was too ill to attend court and may suffer a "catastrophic reaction" if he did. Lord Janner was not present in court to hear today's initial ruling.
The former Labour MP suffers from severe dementia and a decision by Alison Saunders, the Director of Public Prosecutions, ruled he was too ill to face trial in April. That decision was overturned by an independent review.
Chief magistrate Howard Riddle said Lord Janner, who faces 22 charges of child sex abuse spanning three decades, said he was required by law to attend.
The court was adjourned to allow lawyers to determine when he might appear. A Crown Court judge is now expected to decide whether Lord Janner is fit to face a full trial, or whether he will face a "trial of the facts," where a jury hears evidence against an individual considered too ill for a full trial. In the latter case, there would be no finding of guilt or conviction.
Lord Janner and his family denies claims he used his power as an MP to abuse vulnerable young boys at a local Leicestershire children’s home.
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Ms Saunder's initial decision not to charge Lord Janner, despite finding "sufficient evidence" to prosecute him on 22 offences of historic paedophilia, outraged child abuse campaigners and led to the CPS granting an independent review after six of the complainants made a request under a scheme introduced two years ago giving victims the right to challenge CPS decisions.
Director of Public Prosecutions Alison Saunders decided not to charge Lord Janner in April due to the severity of his dementia. Her decision was overturned in June
Delivering his decision, Mr Riddle said Lord Janner "may well become intolerant of the process, irritable and may indeed leave". But he added: "I further understand, and this is very significant, it is likely to have no long-term effect on him.
"He must appear for a comparatively short period of time. He is free to go if he becomes distressed. This will probably be achieved in less than a minute. Nevertheless the law requires his presence."
After Ms Saunders' decision was overturned in April, the Labour MP Simon Danczuk, who has campaigned on behalf of child abuse victims, described it as the latest in a "catalogue of errors from Alison Saunders". He accused her of having "brought the criminal justice system into disrepute".
Earlier Mr Danczuk used parliamentary privilege to claim Lord Janner abused children in Parliament. He said Leicestershire police had told him how the peer "violated, raped and tortured - some in the very building in which we now sit".
He also revealed that he had urged Ed Miliband to throw Lod Janner out of the party five months before the party eventually suspended him. He sent a letter to the former Labour leader in October last year with details of the "stomach-churning" evidence disclosed to him by senior members of Leicestershire police.
At the time her decision was overturned, Ms Saunders said: Ms Saunders said: : "It is a matter of real regret that prosecutions weren't brought by the police and CPS in the past.
"We have brought forward this decision as public speculation on this review has brought damaging and unacceptable uncertainty for the complainants in this case.
"We have now done what we can to inform the complainants of the outcome of the review and I have offered to meet each of them should they wish."
