Sinn Féin leader Mary Lou McDonald has described the leaking of the report into the mother and baby homes as "grossly insensitive" and "unacceptable".
She said the lives of those who lived in the homes were "marked and marred" by their experience and the information contained in the report should not have been released before survivors were briefed on its contents.
It is due to be published by the Government this week but details were reported in the Sunday Independent.
Taoiseach Micheál Martin is set to make a formal State apology to survivors of the mother and baby homes in the Dáil on Wednesday.
The report will contain an estimate that 9,000 children died in 18 institutions between 1922 and the closure of the last such home in 1998.
It will focus on 14 mother-and-baby homes and four county homes.
Children's Minister Roderic O'Gorman is expected to brief survivors of the institutions about the contents of the report ahead of its release.
Ms McDonald told RTÉ Radio 1's This Week programme: "Those whose lives were marked and marred by this experience know the full extent of the horrors they and their families went through.
"I think it is not just grossly insensitive, it is in fact unacceptable that any part of this report has been leaked in advance of families and survivors having access to the report.
"To be honest, I had a worry even before the leak that so little advance notice would be given to victims and advocates to study what will be a very comprehensive report.
"This situation today certainly makes that worse."
She added that a formal State apology was "long overdue" and "necessary" but she said redress needed to be provided after the publication of the report.