A charity representing survivors of child sex abuse has not been granted official representation at the inquiry set up by the Scottish government.
The inquiry, due to report in late 2019, is looking in detail at the historical abuse of children in care.
Wellbeing Scotland said it is now concerned victims that it has helped will be deterred from giving evidence.
The inquiry said it felt the organisation did not meet the criteria to play a significant role.
Wellbeing Scotland formerly operated as Open Secret and said it represented more than 1,000 survivors of child sex abuse.
It has been told it has not been granted core participant status in the inquiry.
It has appealed against the decision as it want its members to receive legal assistance and financial support and to be able to cross-examine witnesses.
The charity said it is the largest organisation in Scotland specialising in historical child abuse cases so it was imperative that it could give evidence to the inquiry.
Janine Rennie, chief executive of Wellbeing Scotland, told BBC Scotland: "The inquiry has given us no specific reason for their decision.
"We are left wondering whether they're concerned that allowing an organisation with 1,058 survivors on its files might lead to a huge increase in costs, and the length of time the inquiry will take.
"But giving us this accreditation would probably save money, compared to granting legal aid to so many individuals."
Ms Rennie said the new inquiry chairwoman, Lady Smith, had said in January 2017 that if any perpetrator of abuse were mentioned in evidence, he or she would be informed.
The charity said this could deter many survivors from coming forward - so it was vital that they had access to legal advice to support them in giving evidence.
The Scottish Child Abuse Inquiry said "core participants" were are individuals and organisations judged to have "a significant role in all or part of the matters being considered by the inquiry".
A spokesman added: "They are more involved in the process of the inquiry than a member of the public and applications to become core participants are therefore determined on the basis of stringent rules.
"Wellbeing Scotland has made an application, which was rejected because it did not meet the required criteria. They have now submitted a fresh application for core participant status which is currently under consideration."
He added: "Wellbeing Scotland has not been banned from participating in the inquiry. Further, it should be noted that the fact that an organisation is not a core participant in no way blocks it or any individual associated it from participating fully in the important work of the inquiry.
"We continue to actively encourage anyone with relevant information to please speak to us."
The first public hearings are due to begin in May and the inquiry is expected to last four years.
Lady Smith replaced the original chairwoman who resigned in July 2016. Susan O'Brien stood down complaining of government interference.