The Labour politician was said to have arranged for children “in whom he had a sexual interest” to be brought to the Houses of Parliament.
Allegations involving Janner, who died aged 87 in December, are being examined as part of the wide-ranging Independent Inquiry into Child Sexual Abuse, chaired by Justice Lowell Goddard.
Speaking at the first preliminary hearing, counsel to the inquiry Ben Emmerson QC said: “The allegations in summary are that Greville Janner exploited children and perpetrated a full range of sexual offences against them, including what would now be termed in English law as rape.
“In relation to a number of the complainants it is alleged that Janner abused his position as an MP by arranging for children in whom he had a sexual interest to be brought to the Houses of Parliament.”
The alleged offending was said to have taken place at children’s homes and hotels between 1955 to 1988, the inquiry heard.
Director of Public Prosecutions Alison Saunders originally decided that the peer, who had denied the allegations, should not be charged because he was suffering from dementia.
However, this was overturned by an independent review and, although Janner was unfit to stand trial, a proceeding called a trial of the facts was due to take place at the Old Bailey in London. This move was abandoned when he died.
Mr Emmerson said that, of more than 30 alleged victims who made contact with police, 12 had been selected to give evidence at trial.
Seventeen complainants have been given core participant status at the inquiry.
The inquiry is unable to determine criminal or civil liability, but Mr Emmerson said it will “often be required to make findings of fact on allegations which, if true, may amount to the commission of a criminal offence”.
Mr Emmerson said the inquiry has so far not received an application for core participant status from Janner’s family. They have previously said he is “entirely innocent of any wrongdoing”.
Referring to the inquiry as a whole, Mr Emmerson said: “The task is vast but critically important.”
The examination of claims linked to Janner – made a life peer by Labour prime minister Tony Blair in 1997 – is one of 13 investigations launched by the inquiry, which is set to last five years and has a budget of £17.9 million for this financial year.
The first full hearings are expected to be held in September.