Organisers of a Catholic women's conference in the Vatican have claimed former president of Ireland Mary McAleese has been denied access.
The Voices of Faith group said the former head of state was one of two names not approved after a list of attendees was sent to a senior cleric in the Holy See.
The Why Women Matter conference, in its fifth year, is the only event linked to International Women's Day (IWD) ever held at the Vatican.
A spokeswoman for the group said they still had no explanation why Mrs McAleese was not cleared to take part.
Chantal Goetz, executive director of Voices of Faith, said: "It was disappointing to realise on IWD of all days, these women who have accomplished so much in their communities, could be turned away from respectfully sharing their stories and experiences as Catholics.
"This event allows us to not only celebrate the wonderful work Catholic women are doing across the globe, but also create discussion and dialogue on the current power and leadership structures of our Church today."
It is understood Mrs McAleese has written to the Pope on the issue.
It is not unusual for a senior Vatican official to vet a list of attendees and speakers for conferences.
Voices of Faith was set up to empower and advocate for Catholic women to be involved in decision making in the church.
It sent a list of names of speakers and attendees for its March 8 conference and a list was sent back with two names not included - Mrs McAleese and Ssenfuka Juanita Warry, who runs a non-profit organisation in Uganda, where to be identified as gay or lesbian can be life-threatening.
Voices of Faith have decided to move the conference from the Casina Pio in the Holy See to the Jesuit Curia in Rome, outside the Vatican.
Mrs McAleese was originally asked to take part in a panel discussion but has since been asked to be the keynote speaker.
A spokeswoman for Voices of Faith said it was in correspondence with Cardinal Kevin Farrell over Vatican approvals for attendees but that it did not receive an explanation for the two women not being on the returned list.
The women's group states that it believes a key solution to many problems facing the Catholic Church lies in diversity of thought, expertise and education at the leadership level.
They state that women and lay people can bring these skills if they were more welcomed.
The Vatican press office, the Catholic Church in Ireland and Cardinal Farrell, the Prefect of the Vatican Dicastery for Laity, the Family and Life, did not respond to request for comments.