A grapefruit cutter is a knife in the eyes of the law and cannot be sold to under-18s, a judge has ruled.
Royal warrant store WJ Daniel & Co, in Windsor, was prosecuted by trading standards after a teenager bought one.
Magistrates in Bracknell, Berkshire, had decided the knife, designed to scoop out grapefruit flesh, was a "food preparation gadget".
But the High Court ruled it was a knife and ordered the decision to be quashed and for the case to be sent back.
The test case was brought to court by Royal Borough of Windsor and Maidenhead trading standards officers seeking a change in the law.
They said if the decision had stood it would have meant young people with malicious intent could have legally bought sharp-bladed kitchen implements.
The store said it marketed the product on the advice of the manufacturer.
It was prosecuted under the 1988 Criminal Justice Act, as amended by the 1996 Offensive Weapons Act.
The £2.99 knife, which has a four-inch long pointed blade serrated on both sides and curved at the end, was sold to a 15-year-old in February 2009, during an undercover investigation.
The 1988 Act gives no definition of what is a knife, and three magistrates ruled it was not a knife under the legislation.
When the case went to court in November, magistrates stated it was designed for a single use and was described in the suppliers' marketing material as "a gadget".
Overturning the decision, Sir Anthony May, president of the Queen's Bench Division, ruled: "An article designed for a single use may well be a knife, and some articles that curve may be knives.
"The suppliers' understanding is not determinative."
Magistrates will have to decide whether the store is entitled to rely on the defence it took all reasonable precautions and exercised all due diligence to avoid commission of the offence.
Steve Johnson, trading standards manager, said it would have set a "dangerous precedent" if the ruling had not been overturned.
Trudy Durkin, managing director of WJ Daniel & Co, said it was up to the local authority to determine whether it would be appropriate to mount another prosecution.
"Daniel has been trading for over 100 years and we pride ourselves in offering the pinnacle of customer service," she added.
"It is significant that this particular gadget has also been marketed on the same basis by most large retail stores and there has never been any suggestion that they are exposed to prosecution."