More than 400 cheap imitations were collected following raids in Harrow and Oxford by the Medicines and Healthcare products Regulatory Agency (MHRA).
Bogus digital thermometers are often sold online for just 99p and can seriously endanger health, particularly among those with potentially killer illnesses.
The MHRA launched an investigation after the parents of a leukaemia sufferer were forced to rush their child to hospital for urgent care despite an apparently normal temperature reading.
The child, in fact, had a high temperature which was not picked up by the thermometer they bought on the internet.
Dr Nicola Lennard, deputy clinical director at the MHRA, said: "Inaccurate readings from cheap, fake thermometers could result in a delay to a child getting the medical treatment they need."
The agency is working with eBay to prevent the sale of unapproved devices.
The fake thermometers have no recognised brand name and often lack the requisite safety markings, warnings and instructions for use.
Anyone with a faulty device is urged to contact the MHRA's hotline on 020 3080 7080 or go to its website at www.mhra.gov.uk