The death of Lord Janner is "extremely sad" both for his family and for the alleged victims of sexual abuse who will be denied justice, a campaigner said.
Labour MP Simon Danczuk, who led calls for the peer to face justice, spoke out after the family announced he had died peacefully at home aged 87.
A High Court judge ruled this month that Lord Janner's "deteriorating and irreversible" dementia meant he could not stand trial in person.
But a "trial of the facts" was scheduled for April to examine charges related to 22 sexual offences dating back to the 1960s against nine alleged victims, mostly under 16 at the time.
The Crown Prosecution Service indicated that the death of Lord Janner would mean it was halted, even though he would not have been giving evidence.
Mr Danczuk said: "Obviously it is very sad for Lord Janner's family that he has passed away, though it is also extremely sad for his alleged victims.
"The CPS saying that the trial of the facts will not now go ahead will be very disappointing."
Mr Danczuk, who has pursued several high-profile cases of alleged child sex abuse, said the allegations against Lord Janner "were some of the most horrific examples that I've heard".
"It is disappointing that all the evidence is not now to be brought before a court," he added.
"It is a very sad day for justice."
Mr Danczuk suggested the allegations were now likely to be examined by the independent child abuse inquiry being led by Justice Lowell Goddard.
Liz Dux, abuse lawyer at Slater and Gordon, who represents six of Lord Janner's alleged victims, said: "This is devastating news for my clients. They have waited so long to see this case come before the courts, to be denied justice at the final hurdle is deeply frustrating.
"Worse still is that so many opportunities to bring this matter to justice whilst he was well were missed.
"All they have ever wanted is to give their evidence in court and have these very serious allegations tested and to be believed."
Lord Janner was charged with 22 sexual offences dating back to the 1960s against nine alleged victims, the majority of whom were 16 or under at the time.
There are 15 counts of indecent assault and seven counts of a separate sexual offence said to have taken place in the 1960s, 1970s and 1980s. Twenty-one of the charges relate to children who were aged 16 or under at the time.
Director of Public Prosecutions (DPP) Alison Saunders had ruled that Lord Janner should not be charged for alleged child sex crimes because of his ill health.
But that decision was overturned by an independent review in April.
In a trial of the facts, a jury considers the evidence against an individual but there is 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.
Sir Mick Davis, chairman of the Jewish Leadership Council which was one of many organisations championed by Lord Janner, said: "The passing of Greville Janner marks the end of an era for the Jewish community.
"Our thoughts and prayers are with the Janner family at this most difficult time."
The Holocaust Educational Trust also paid tribute to Lord Janner. Its chief executive, Karen Pollock, said: "In 1988 Greville Janner had the foresight to know we as a nation needed to know about and remember the Holocaust and so established the Holocaust Educational Trust. Our thoughts are with his family at this difficult time."