Leyton school bans Muslim children fasting for Ramadan
In a letter to parents, Barclay Primary School in Leyton said it has previously had a "number of children who became ill and fainted", or been unable to learn while fasting.
The school said it had "sought guidance" and that children were not required to fast during the period.
Critics said the move was an overreaction and unnecessary.
Ajmal Masroor, Imam and spokesman for the Islamic Society of Britain, said children were not expected to fast, especially not primary school children.
He said: "It could have been easily resolved by just speaking to parents. Now we have another negative story against the Muslim community - as if we don't have enough already."
The action was "stupidly foolish", he said.
Barclay School said the month fell during a hot spell and a busy time of the year with sports days and field trips.
It suggested older children who wanted to take part in fasting "do so at weekends".