Alisha Frost, 24, boarded the bus with her two-year-old son Jack, who suffers from cerebral palsy, in Pontypridd, Wales.
But, when she pushed her son's specially-adapted pushchair to the disabled area, she discovered that the seats had already been taken by passengers in their late 60s and 70s.
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She said: "They told me they didn't have to move for me and told me to just fold the buggy up and sit my son on my knee.
"I explained that I couldn't do this as my son is disabled and he needs the correct support or he flings himself forwards."
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Apparently the bus driver - who initially refused to help - eventually intervened, but the frustrated mother was still forced to sit on the floor and tightly cling on to her son's pushchair as she did not have enough room to put the brake on.
Unsurprisingly, she left the bus in tears after no one helped her.
She said: "I'm furious about the whole situation and have been so upset since - it's just wrong that we were treated in this way."
A spokesman for Stagecoach in South Wales said they were investigating the incident.
They said: "All of our drivers receive formal customer care training, so it is extremely disappointing to be told of this lady's and her child's experience, which falls far below the standard that we expect and regularly achieve from the vast majority of drivers employed by us."