Harrison Catering has been ordered to improve its meals service by a north London primary school.
In a letter to the contract caterer, governors of the St Paul's Church of England primary school in Camden, outlined concerns about the quality of the food and staffing problems.
"The portion sizes are very small," said one parent. "There's too much processed and fried food on the menu and vegetables are overcooked."
They added that they were "disappointed and astonished", because they had heard positive reports about Harrison before it took over from Compass division Scolarest in September.
Another parent said that although meal prices rose from £1.50 to £2 when the contract changed hands, the service was now "marginally worse".
But Harrison, which was given until Monday 17 October to respond to the warning, claimed it had not received the letter. A spokeswoman for the company said: "There's no evidence from any of our schools to suggest that children are served small portions.
In addition, there's no evidence of poor cooking, and we're not aware of any official complaint to imply otherwise."