Speaking for the first time, she said: "The girl was in a cubicle. She had her jeans on but there was blood all over the floor. We thought she was haemorrhaging. Someone noticed there was blood in the next door cubicle, and when we went in there was a baby there, too, in a sanitary bucket.
'We picked it up and one of the workers gave it mouth-to-mouth.
"But all the girl kept saying was ... I've got to go back to work, I've got to go back to work .... She had had the baby and cut the cord with a box cutter."
The infant was given first aid, an ambulance was called and it later recovered in hospital.
The mother, now 33, was arrested and released without charge. It is believed the child is no longer in her care.
Severn added that her supervisor and employment agency had said they were not aware the woman was pregnant.
In 2016 MPs heard that Sports Direct staff were so scared of losing their jobs they came to work heavily pregnant, although it is not known the circumstances in which the mother came to work. And the company has denied that anyone turned up to work when they were unfit to do so.