The EU wants to classify barley straw as a biocide, meaning that manufacturers will have to spend thousands of pounds proving it is safe to use. This could make it too expensive to carry on producing.
The natural product, used by gardeners to keep pond water clear, will come under the full force of EU regulation in September.
The Ornamental Aquatic Trade Association (OATA) described the decision as “bonkers”.
It said water companies put tonnes of barley straw into reservoirs every year to control algae – a practice that will not be covered by the new law.
OATA boss Keith Davenport said: “We couldn’t believe it when we saw barley straw had been included in the regulation.”
Barley straw could disappear from garden centre shelves this autumn.
Water gardening expert and author Graham Clarke said: “This is bad news for thousands of pond-owners. Soon they’ll stop us from calling them goldfish, unless they’re made of real gold!”