Traces of a toxic chemical have been found in counterfeit versions of Sensodyne toothpaste on sale in the UK.
The fake toothpaste has both English and Arabic writing
The fake batch is being sold at markets and car boot sales, but not in shops, said the Medicines Regulatory Agency.
The fake tubes have both English and Arabic lettering, while the real ones use only English in the UK.
Sensodyne's owner, GlaxoSmithKline, said the authorities had seized 140 of the counterfeit tubes, which were first found at a car boot sale in Derbyshire.
The chemical in question, diethylene glycol, can be toxic to children or people with impaired livers or kidneys.
In a statement, the health watchdog said that while it did not yet know of anyone being affected, all fake tubes should be destroyed.
"These fake products are illegal and have no connection with GSK," the company said in a statement.
The affected products are Sensodyne Original and Sensodyne Mint sold in 50ml tubes.
Their batch code is PROD 07 2005/EXP 08/2008.
GSK added that it did not use diethylene glycol (DEG) as an ingredient in any of its toothpastes.
The firm said it was not yet known where the counterfeit product originally came from, but that China was "in the frame" as it had recently exported toothpaste contaminated with DEG.
A number of batches of contaminated fake toothpaste from China were discovered in the US recently, causing a health scare.