A Catholic archbishop in the Republic of Ireland has said the church has no records about the burial of nearly 800 children at a mother and baby home.
The children, aged between two days and nine years, died between 1925 and 1961.
The remains of some of the children were in a disused concrete tank at the County Galway home.
The grave in Tuam was found nearly 40 years ago, but was initially thought to be from the 1850s famine.
Archbishop of Tuam Michael Neary said he was "greatly shocked" by the news.
"I was greatly shocked, as we all were, to learn of the extent of the numbers of children buried in the graveyard in Tuam.
"I was made aware of the magnitude of this situation by media reporting and historical research.
"I am horrified and saddened to hear of the large number of deceased children involved and this points to a time of great suffering and pain for the little ones and their mothers."
The home was run by nuns of the Bon Secours Sisters.
Archbishop Neary said that regardless of the time lapse involved it was a matter of great public concern that "ought to be acted upon urgently."
"As the diocese did not have any involvement in the running of the home in Tuam, we do not have any material relating to it in our archives.
"I understand that the material which the Bon Secours Sisters held, as managers of the mother and baby home, was handed over to Galway County Council and the health authorities in 1961.
"While the Archdiocese of Tuam will cooperate fully, nonetheless there exists a clear moral imperative on the Bon Secours Sisters in this case to act upon their responsibilities in the interest of the common good."
He said he would make it a priority to work with the families of the deceased, to obtain a "dignified re-interment" of the remains of the children in consecrated ground in Tuam.
Another congregation of nuns, which ran three mother and baby homes, has said it would welcome an independent inquiry into the burial of babies and children in unmarked graves.
The Sisters of the Sacred Hearts of Jesus and Mary ran homes in Sean Ross Abbey in County Tipperary, Bessborough near Cork city and Castlepollard, County Westmeath.
In a statement to RTÉ News, a spokesperson for the congregation said they would be happy to take part in such an inquiry to establish the truth about what it called a "very sad chapter in the history of Irish society".
'Scandal of significant proportion'
Fianna Fáil TD (member of parliament) Colm Keaveney, whose home town is Tuam, said the burial of the children in a septic tank was "horrendous" and a "scandal of significant proportion".
"I've called on the government to make a formal apology to the women involved and take whatever action necessary to unearth the truth," he said.
"We need to hear a formal statement from the taoiseach (prime minister) of this country about plans to investigate the circumstances surrounding the death of these children.
"These infants were Irish citizens, their treatment and the treatment of their mothers, was grossly unacceptable."
The remains were originally thought to be those of victims of the Irish famine, however, local historian Catherine Corless found that the register of deaths and burials in the town did not match.
"I went to the births, deaths, marriages registration office in Galway and I asked them would they have records of the children who died at the home," she told the BBC.
"When she came back to me, she said, 'We have the records... but there's quite a number.'"
"I was staggered and I was shocked because there's a total number of 796 babies, children and toddlers buried in one mass grave there on that site."
Funds are now being raised to erect a permanent memorial to the dead children.
Ireland's Catholic Church has recently been affected by a series of allegations of abuse and neglect of children who were in its care.
"Many of the revelations are deeply disturbing and a shocking reminder of a darker past in Ireland when our children were not cherished as they should have been," said Children's Minister Charlie Flanagan.
"I am particularly mindful of the relatives of those involved and of local communities."
The Tuam home was one of 10 institutions in which about 35,000 unmarried pregnant women - so-called fallen women - are thought to have been sent.
County Galway death records showed that most of the children buried in the unmarked grave had died of sickness or malnutrition.