Traces of a toxic chemical have been found in dozens of tubes of fake toothpaste seized at a car boot sale in Derbyshire, a medicine watchdog said.
Traces of a toxic chemical have been found in counterfeit versions of Sensodyne toothpaste on sale in the UK.
The affected batch was not sold in shops, the Medicines and Healthcare Products Regulatory Agency (MHRA) said.
The fake batch is being sold at markets and car boot sales, but not in shops, said the Medicines Regulatory Agency.
All tubes of the Sensodyne toothpaste contaminated with toxic levels of diethylene glycol should be discarded.
The fake tubes have both English and Arabic lettering, while the real ones use only English in the UK.
But the watchdog has "no evidence" yet of any UK consumers suffering adverse reactions to the fake toothpaste.
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 fake packs are illegal and have no connection with Sensodyne's manufacturer GlaxoSmithKline.
The chemical in question, diethylene glycol, can be toxic to children or people with impaired livers or kidneys.
The fake toothpaste was sold in packaging written in both English and Arabic.
'No connection'
Genuine Sensodyne packs made by GlaxoSmithKline have packaging written only in English.
In a statement, the health watchdog said that while it did not yet know of anyone being affected, all fake tubes should be destroyed.
Diethylene glycol
"These fake products are illegal and have no connection with GSK," the company said in a statement.
Tests on the seized toothpaste showed a higher than safe level of a chemical called diethylene glycol which is used at very low levels in some other toothpastes.
The affected batch code is PROD 07 2005/EXP 08/2008.
The affected products - fake Sensodyne Original and Sensodyne Mint sold in 50ml tubes - were sold via "unauthorised" suppliers and not legitimate shops.
GSK added that it did not use diethylene glycol (DEG) as an ingredient in any of its toothpastes.
Diethylene glycol could be toxic to young children and anyone with an impaired liver or kidney function, the MHRA warned.
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.
"We believe that elderly patients (possibly with impaired liver and kidney function) may have purchased this stock due to lower prices in markets, discount shops and car boot sales," the MHRA warned.
A number of batches of contaminated toothpaste were discovered in the US recently, causing a health scare.
The MHRA's inquiries into the incident are continuing and it has asked hospitals, wholesalers and pharmacies to check for Sensodyne in combined Arabic and English packaging.
Both the genuine Sensodyne Original and the fake toothpaste have a pink coloured paste, but the Genuine Sensodyne Mint toothpaste is white in colour whereas the fake version has a green paste.
Last month, the US Food and Drug Administration issued a warning after diethylene glycol - which is used in antifreeze - was found in several brands of toothpaste imported from China.
