The Medicines and Healthcare products Regulatory Agency (MHRA) is working with online retailers to remove Nutrition Ignition Kids Magnesium Glycinate Gummies from sale.
Magnesium glycinate is a dietary supplement that can help with anxiety and improve sleep.
However, tests on two batches of this specific brand showed between 1.5 and 1.7mg of melatonin in each individual gummy.
Melatonin, the hormone that helps regulate the sleep-wake cycle, is naturally produced by the body.
However, a synthetic version is authorised for use on prescription in the UK for adults and children over the age of six with sleep disorders such as insomnia.
If prescribed, the recommended starting dose is 1mg for children.
According to the MHRA, melatonin is not listed anywhere on the packaging of Nutrition Ignition Kids Magnesium Glycinate Gummies.
The packaging advises a dose of one to two gummies per day.
Taking too much melatonin can cause headaches, drowsiness, dizziness and nausea.
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Dr Alison Cave, chief safety officer at MHRA, said: "We advise any parent or caregiver to stop use of this product and safely dispose of it.
"Side effects such as headache, hyperactivity, a feeling of dizziness and abdominal pain have been reported in children when melatonin is prescribed and used for its licensed indications. No serious side effects were observed in studies in children.
"Anyone who suspects that their child, or a child in their care, is having a side effect from this product is advised to stop taking it and speak to a healthcare professional and report it directly to the MHRA Yellow Card scheme."
Nutrition Ignition Kids Magnesium Glycinate Gummies are sold online as a food supplement, but the MHRA has now removed the product from sale and is working with online retailers to remove all listings.