WILLIAM Tyrrell's biological mother is a young woman called Karlie Tyrrell, whose identity has been kept secret for three years because of child protection laws.
But news.com.au can now reveal her name following the lifting of legal bans.
This follows a legal court challenge to laws which forbade even revealing that the missing boy had been fostered out at the time of his September 12, 2014 disappearance,
Following three-year-old William's apparent kidnap from his foster grandmother's house at Kendall on the NSW North Coast, Karlie Tyrrell flooded Facebook with grieving posts.
The young mother posted pictures of herself pregnant with William, and with a young man believed to be the biological father of William.
As the agonising search for missing William stretched into months and then years, Karlie Tyrrell still kept the memory of her missing son alive.
William Tyrrell's parents make desperate plea for missing toddler
Hoping desperately that he would be found, Ms Tyrrell nevertheless was unable to go public with appeals to find her missing child.
The case of William Tyrrell is one of Australia's most puzzling missing child cases.
There's no suggestion that either William's biological or foster families were involved in his disappearance or have any knowledge of his whereabouts.
But it had been their and the Department of Family and Community Services (FACS)'s secret until Thursday, that William was being cared for by foster parents.
In 2016 there were almost 700 people in the sights of the investigators and in September 2016, a $1 million reward for information on William's disappearance was announced.
At the time William disappeared from the house in Kendall, he was under the control of the Department of Family and Community Services (FACS), being cared for by foster parents in what is referred to as out-of-home care.
William would have turned six years old in June, and NSW Police have grave fears for his welfare.
The secret details of his status as a foster child were made public by Channel 9's A Current Affair program after statutory restrictions were clarified by a judge.
William's biological father had struggled continually with drug and alcohol problems and life behind bars, ACA said.
NSW Court of Appeal documents reveal a Supreme Court judge found the fact William was in the parental responsibility of the NSW Minister for FACS and in the care of departmentally approved carers, was a matter of "legitimate public interest".
Justice Paul Brereton noted "the tragic probability that (William) is no longer alive".