A member of the public alerted local councillors to the problem at the Tesco Express store after claiming to have seen customers who looked under-age buying alcohol at the self-service tills without being asked for ID.
Tesco initially admitted to the "IT error", saying a fault with the system when the checkouts were being updated meant that no authorisation was requested when a customer attempted to buy an age-related product.
But the supermarket has since denied there was a problem. A spokesman said: "Initial indications seemed to suggest there was an IT error which allowed this to occur, however, subsequent independent checks shows there seems not to have been a problem at all.
"The likelihood of this happening is remote, every alcoholic product which is scanned will not be processed until a
staff member authorises the purchase."
Standards officers in Monmouthshire will not be taking action against the Tesco Express store because it cannot prove than any under-18s purchased alcohol during that time.
Gareth Walters of Monmouth Trading Standards said: "We have since run
test purchases at the store and each time it has requested identification."