Two babies have died of an E.coli infection, health officials have confirmed.
Both cases are under investigation at Singleton Hospital, Swansea, south Wales.
One of the babies was "very premature" and died in hospital.
The second case involved a baby who died in the community but whose mother is suspected of contracting the infection at the hospital.
Abertawe Bro Morgannwg (ABM) University Health Board runs Singleton Hospital. It is investigating both cases of ESBL E.coli cross-infection in the hospital's maternity/neonatal unit, supported by Public Health Wales.
Health chiefs at the hospital stressed today that ESBL E.coli is not the same as E.coli O157 which causes food poisoning.
In most people ESBL E.coli does not cause harm but in vulnerable individuals it can cause serious infections. Action has been taken by hospital chiefs to restrict use of the unit to full-term babies.
Dr Bruce Ferguson, medical director of ABM University Health Board, said: "Tests have confirmed that in one of these cases the ESBL E.coli infection was contracted in the hospital.
"Sadly, this was a very premature baby who, despite the best efforts of staff, later died. The cause of death of this baby is currently being investigated by the coroner. Everyone involved with the unit and in the care of this baby deeply regret this tragic loss.
"The second case involved a mother who tested positive for ESBL E.coli but who has not shown any symptoms and has not required treatment. We suspect she also contracted ESBL E.coli within the hospital, but this can only be confirmed in test results which we expect later this week."