This raises the possibility that the investigation into the singer's death may have to be held again.
Winehouse's relatives said they were still absorbing the implications of the news and were seeking legal advice.
In October, Suzanne Greenaway ruled that the soul singer had died from accidental alcohol poisoning.
Greenaway was appointed as an assistant deputy coroner in London in 2009 by her husband, coroner Andrew Reid.
She resigned in November after authorities found out she had not been a registered U.K. lawyer for five years as required. She had practiced law for 10 years in her native Australia.
"I appointed my wife as an assistant deputy coroner as I believed at the time that her experience as a solicitor and barrister in Australia satisfied the requirements of the post," he said in a statement Wednesday. "In November of last year it became apparent that I had made an error in the appointment process and I accepted her resignation."
Greenaway oversaw 12 inquests in Camden, the north London borough where Winehouse lived, and others in eastern London.
Reid said he was "confident that all of the inquests handled were done so correctly" - but offered to hold the inquests again if the families of the deceased wanted it.
Winehouse's family said it had not yet decided what to do.
In a statement, the family said it was "taking advice on the implications of this and will decide if any further discussion with the authorities is needed."
The 27-year-old soul singer was found dead in bed July 23 at her Camden home.
At the October inquest, Greenaway delivered a verdict of "death by misadventure," saying the singer suffered accidental alcohol poisoning when she resumed drinking after weeks of abstinence.
The Associated Press contributed to this report.