Buckinghamshire Trading Standards ran the sting operation in May and found teenagers could purchase knives with ease.
The sting was part of nationwide test purchasing at national retailers by trading standards bodies. They found that 28% of the under-age children used in the operation were able to purchase knives from the retailers. The minimum age for buying knives is 16.
Bucks Trading Standards safety team leader Nick Heirons said: "I was surprised at the ease with which the teenagers could purchase knives."
He added that retailers across the country could expect more test purchasing of knives by under-age children in the future.
A Morrisons spokesman said: "Store staff are trained to be highly vigilant in the sale of knives to children. This was an isolated incident within Morrisons."