Lord Janner, the former Labour peer and MP ruled unfit to stand trial on child sexual abuse charges, has died.
He had been suffering from dementia and died peacefully at his home on Saturday aged 87, his family said.
The peer had been accused of 22 counts of historical sex offences against boys - allegations his family denied.
A "trial of the facts" set for April will no longer take place. A lawyer representing alleged victims said they had been denied justice.
Liz Dux, who represents six 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."
The Goddard inquiry, which is examining child sexual abuse claims, may now examine the allegations, the BBC's Tom Symonds said.
It had set aside the case while it was being dealt with in the courts.
'Trial of facts'
Lord Janner's family said he would be "deeply missed" and requested their mourning be respected.
They had previously said he was a man of "great integrity" and "entirely innocent of any wrongdoing".
Labour MP Simon Danczuk, who led calls for Lord Janner to face justice, 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."
He added: "It is disappointing that all the evidence is not now to be brought before a court.
"It is a very sad day for justice."
By Chris Mason, BBC political correspondent
Should a man with what was described as "advanced and disabling dementia" face a trial over allegations of child sexual abuse spanning four decades?
It is that question - and the conflicting answers offered to it - that places Lord Janner's death so prominently in the news.
A High Court judge, just weeks ago, said his answer was no: there was little point in hauling a man who could barely recognise his own family in front of a jury, to answer for his alleged behaviour a generation ago.
That was galling enough for his alleged victims.
But they sought solace in the so-called "trial of the facts" due to begin in the spring.
That will now not happen - leaving them devastated, and so many questions about Lord Janner's alleged behaviour permanently unaddressed.
The peer had been on a leave of absence from the House of Lords since October 2014.
He was accused of 15 counts of indecent assault and seven counts of a separate sexual offence against a total of nine alleged victims.
The Old Bailey heard the allegations spanned the years 1963 to 1988.
The judge who ruled that he was unfit to stand trial earlier this month said he made his decision based on four experts.
"The defendant has advanced and disabling dementia that has deteriorated and is irreversible," Justice Openshaw said at the time.
Who was Lord Janner?
- Born in Cardiff in 1928
- Served in the Army and studied at Cambridge before becoming a barrister and then QC
- Labour MP for Leicester North West and then Leicester West from 1970 until retiring in 1997, when he was made a life peer
- Diagnosed with Alzheimer's disease in 2009
- Suspended from the Labour Party in April
- Ruled unfit to stand trial over allegations of child sexual abuse in December
Born in Cardiff, he was the grandson of eastern European Jews who fled to Britain.
After being evacuated to Canada during the Second World War, he returned to Britain to complete his education before joining the Army.
He was first elected to the House of Commons in 1970, winning the seat of Leicester North West after his father's retirement from representing the seat.
As a backbencher, he campaigned for Israeli and Jewish causes, as well as on consumer safety, women's equality and workers' rights.
And in the 1980s, he helped to launch the Parliamentary War Crimes Commission, set up to report on ex-Nazis living in Britain.
Sir Mick Davis, chairman of the Jewish Leadership Council, 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. 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."