The Historical Institutional Abuse Inquiry is examining allegations of abuse in state-run children's homes and other institutions in Northern Ireland.
However, victims of clerical child abuse and former residents of Magdalene laundry-style institutions in Northern Ireland are excluded from the remit.
They have launched a joint campaign.
The two groups were due to call for the remit of the Historic Institutional Abuse Inquiry to be widened.
The inquiry, chaired by retired judge Sir Anthony Hart, was set up by the Stormont Executive and is examining cases from 1922 to 1995.
It will also determine if there were systemic failings by the state, or institutions, in their duties towards children under 18.
It was originally set up to investigate cases going back as far as 1945, but it was later extended to cover the period back to the foundation of the state in 1922.
Previously, Sir Anthony has opposed further extending the scope of his inquiry to deal with abuse which took place outside institutions.
He pointed out that such a move would have significant implications in terms of time and money.
The inquiry is currently investigating 35 residential facilities.
They include 15 state-run children's homes, 13 institutions run by Catholic Church orders and four borstals or training schools.
The inquiry was announced in December 2010 and the first phase began in October 2012.
Three institutions run by Protestant churches or voluntary organisations will also be investigated.
Abuse victims whose cases are excluded from a Stormont inquiry into historical child abuse have united in a campaign for their allegations to be included.