The Pesticides Residue Committee said the white seedless fruit from the US contained 0.2mg/kg of methomyl. The maximum residue level for this chemical is 0.05mg/kg.
The PRC said it was unable to name the retailer concerned until it had responded. It said the residue posed no risk to health and the Food Standards Agency had agreed with its assessment.
The PRC tested 23 samples of grapes - 14 contained residues, nine of them more than one.