Lynn Harrison was pictured carrying Tammy, 6, towards the finish line in a moving photo.
She has cerebral palsy and suffers from poor co-ordination, stiff muscles, and tremors.
However, Lynn was afterwards asked to leave by members of staff at Jeavons Wood Primary School in Cambourne, Cambridgeshire.
She told Cambridgeshire Live: “When I got to the school they said to me I can’t help Tammy.
“They told me I couldn’t help her because of insurance and child protection, so obviously I wondered and asked how she was going to take part.
“Last year she was able to come out of the buggy and walk with me carrying her. When she does that she feels like she’s joining in and she enjoys it more.
“It also gives her more exercise which is really good for her.
“But they told me [if she was to take part] her one-to-one would have to push her around in her wheelchair buggy.”
Despite the fact Lynn had done the same in 2018, staff refused because of “child protection”.
She added: “I think it’s absolutely disgusting from the school.
“They told me I couldn’t help her because of insurance and child protection”
“If I wasn’t there, she would have had to stay in her buggy and imagine how she felt, sat in her buggy thinking ‘look at all my peers and I am left here sat in my wheelchair.’
“Her one-to-one even gave her the option and she said I want mummy to help me do it. But the school never gave her that option.”
Tammy was pictured beaming as she crossed the finish line before staff intervened.
Lynn later posted a status on Facebook in case any parents had been offended by her actions, with one responding that she’d done the right thing.
Prior to the incident, she was reportedly called in to help Tammy go to the toilet when the school was short staffed.
“During sports day I made a point and said to the teachers ‘it’s ok for me to do that but now it isn’t ok for me to help out’”, she continued.
“This is down to discrimination.
“I simply don’t get it if she is able to do sports day with my help then what’s the problem?”
Headteacher Sue Wright said in a statement: “If a parent is upset by a decision taken by the school then we have procedures in place to address those concerns.
“We always try and solve any issues informally but there is a clear complaints procedure to follow if they are not resolved.
“No parent has contacted the school informally or formally to make a complaint about sports day.”