Premature twins died after nurses gave them a morphine dose of at least 10 times recommended levels at the Stafford Hospital, a disciplinary hearing has been told.
Alfie and Harry McQuillan, who were born by Caesarean section at 27 weeks, were given the drug in error and in breach of strict protocols.
When the nurses found out what had happened, one of them became 'absolutely hysterical, and kept saying, 'oh my God, what if I've killed these babies?''.
The Nursing and Midwifery Council (NMC) heard that Joanne Thompson, a senior nurse in the neonatal unit, failed to oversee adequately the administration of morphine before the twins died on Nov 1 2010.
Lisa Lucas, a junior nurse in the special care baby unit, miscalculated the dosages, meaning the boys received 600mg and 850mg of morphine respectively instead of a normal dose of between 50mg and 100mg.
She was aided by a junior doctor, Adina Olariu, who had 'extremely limited experience', in breach of rules that state two registered nurses should deliver the medication The doctor had never administered the drug before.
Plans were made to transfer the babies to a specialist unit at the nearby University Hospital of North Staffordshire in Stoke-on-Trent but they died before arriving.
Another neonatal nurse, Hayley Amos told the hearing that when the mistake was identified, Mrs Thompson 'went very quiet at this point, and was visibly shocked'. Miss Lucas was 'absolutely hysterical, and kept saying, 'oh my God, what if I've killed these babies?' as she cried and shook uncontrollably.
Both nurses were suspended that morning, but Mrs Thompson returned to work just days later before a disciplinary inquiry began. Mrs Thompson, who broke down during the hearing, denies a series of charges including failing to take the clinical lead of nursing care when required in relation to the administration of the morphine to the babies.
An inquest into the twins' deaths last year heard that they were given an 'excessive' dose of morphine hours after their birth.
The Telegraph reports that the coroner, who recorded a narrative verdict, heard that despite being born prematurely, they had been in a 'good' condition after their birth.
The Francis inquiry last month found appalling standards of care at Stafford Hospital, where between 400 and 1,200 people died unnecessary between 2005 and 2009. The misconduct hearing continues.