Hospital bosses have apologised to two mothers after their babies were accidentally swapped.
One of the babies was breast-fed by the wrong mother, and both babies were given checks including an HIV test.
The incident happened at Bassetlaw Hospital in Worksop, Nottinghamshire, last year. Two staff were disciplined.
A hospital spokesman said an investigation had begun into what was an "extraordinary and most unfortunate incident which we deeply regret."
A member of staff at the hospital or a relative reported the incident after the maternity unit was given an "excellent" rating by the Healthcare Commission in its recent report, it is believed.
The error happened after the babies were separated from their mothers - one is believed to be Polish, the other local - for an unknown reason.
When the babies were returned to the mothers they were accidentally swapped before staff realised the mistake.
The trust said it was an hour before staff realised a mistake had been made.
A spokesman for Doncaster and Bassetlaw Hospitals NHS Foundation Trust said: "We can confirm that there was a most unfortunate, isolated incident in which two babies were mistakenly given to the wrong mothers for a brief time.
"Although every baby has an identification band, it is apparent that these were not checked properly with the result that babies were not given to their own mothers and one child was fed once.
"This incident is the subject of appropriate action with the staff involved. Both mothers have had an apology and a full explanation."
Breedagh Hughes, of the Royal College of Midwives, expressed her concern at the incident.
She said: "Mistakes like this quite often... happen at a time when staff are pressurised, when perhaps there aren't enough staff available..."