A B&Q branch is investigating claims that a young disabled child sustained a head injury after falling out of a trolley in the store.
Tiffany Zimbandu's mother says she was in shock after a concussion at the store in Cheltenham.
The four-year-old, who has Down's syndrome and skin condition vitiligo, was rushed to the nearest A&E after the fall.
Her mum, 40-year-old Michelle Owino, told Gloucestershire Live that the tot "cracked her head" and bruised her back badly.
The mother and daughter were sent home from the hospital with a handful of prescribed painkillers and head injuries on a pamphlet.
Doctors at Cheltenham General Hospital told the worried mum to keep a close eye on Tiffany for at least 72 hours.
Mr Owino claimed that the incident would leave Tiffany so terrified of riding in trolleys that she would have to push her around the shops in a wheelchair, making it "very hard" to do her shopping.
She: "You are not happy. I am going to write a letter because she has Down's Syndrome – it is very hard.
"That could have affected her for the rest of her life, in the sense of shock. She feels comfortable going to the trolley now so this makes things very difficult for me.
"It's absolutely disgusting what has happened."
Also suffering with joint hypermobility syndrome – also known as double-jointedness – Tiffany occasionally relies on a wheelchair to get around.
This medical condition affects her balance, and often causes her additional pain.
Her mum was in B&Q, at the Golden Valley Retail Park in the outskirts of Cheltenham, shopping for beading for her laminate floor at the time of the accident.
Ms Owino, a full-time carer for her daughter, strapped her into a baby seat in a trolley.
While her back was turned, the plastic seat snapped, sending Tiffany crashing into the metal trolley bed.
The distressed girl then "screamed" as Ms Owino rushed to her aid.
After the accident, her mum said she was "lucky to be alive".
She said: "She had fallen back. Her head cracked on the bottom of the trolley.
"The back of the seat was ticking up in her back. The plastic bit where she was sitting on her bum was completely snapped away from the metal.
"She cracked her head very hard. She was in terrible shock. You'd expect a seat to give way to a child. It could have killed her."
No staff came to help Ms Owino and her eldest daughter, 16-year-old Chantelle, she accepted.
B&Q has pledged to investigate the cause of the accident.
A spokeswoman for the company said: "We can confirm there was a customer complaint involving a trolley seat at our Cheltenham store on August 13, 2019. Investigating the use of the trolley in order to be able to respond to the complaint."
Ms Owino is in the process of registering a formal complaint with the DIY giant.