All infants in English state schools are entitled to free school meals for the first time
An investigation has been launched after claims reception children in Birmingham were served just one chicken nugget during a school lunch.
Councillor Valerie Seabright said it raised concerns about the quality and portion size of free school meals.
She said she witnessed the meal being offered to reception children during a visit to one school in the city.
Birmingham City Council said it followed government guidelines and was investigating the case.
Ms Seabright raised her concerns about the unnamed school during a council scrutiny committee, the Birmingham Mail reported.
She said the single nugget offered to reception children was "disproportionate", but she had no evidence it was a regular occurrence at the school.
"The idea of free school meals, which I think is a fantastic idea, is that it gives children the opportunity to have what I would hope is a balanced nutritious meal," the Labour councillor said.
"If you're still serving up chicken nuggets I'd be a bit concerned about processed food being on the agenda.
"If I discovered my grandson was receiving these not nutritious foods I would recommend to my daughter she sent in sandwiches."
Birmingham City Council said it took the issue "very seriously" and was working "to establish the facts in this particular case".
In a statement, it said: "Once we know more information we can act accordingly to ensure that the highest standards of nutritious school meal provision is continued to be delivered by our hard working and dedicated team of staff.
"We offer balanced menus and follow all nutritional guidelines laid down by government."
From this month, all state-funded schools in England, including academies and free schools, have a legal duty to offer free meals for all children in Reception to Year 2.
Ms Seabright said it had put many schools "under serious pressure".