Alcohol was sold to an under-age teenager at four licensed premises in West Sussex.
Test purchases at pubs and off-licences in Horsham and Crawley were carried out on May 23 by a 16-year-old plain-clothed police cadet as part of a Sussex Police investigation.
Eight premises in West Sussex were tested and half of them sold alcohol to the 16-year-old – Limeburners in Billinghurst, The Queens Head in West Chiltington, The White Horse Hotel in Storrington and Wine Express on Tilgate Parade in Crawley.
Sergeant Tony Jarred said: “Sussex Police takes this type of incident very seriously and will look to take robust action against any premises who are putting young people at risk of harm.
“Any person caught selling alcohol to a child under 18 will be given a £90 fine or may face prosecution.”
The aim of the operation, carried out by the Neighbourhood Licensing Team, was to identify licensed premises selling alcoholic drinks to under-18s and to take appropriate action.
Sgt Jarred added: “The sale of alcohol to children is totally unacceptable.
“Young people becoming intoxicated is not only a health risk, but also leads to crime and antisocial behaviour which can have a big impact on local communities.”
Sussex Police will be following up this investigation with the four premises and could take action against them, which could involve a fine or the premises losing their licence.
The Queen’s Head was the only establishment that would discuss the incident.
Owner Gemma Kettink said: “This incident was a one-off and was a very traumatic incident for the member of staff involved. As a result we’ve ordered a lot more information on Challenge 21 and are encouraging staff to talk about it more so that this doesn’t happen again.”
The Wine Express, The White Horse Hotel and Limeburners declined to comment.