Dentists have been warned against using a hand-held X-ray machine on patients as it poses a significant health risk.
The cheap imported machine, known as the Tianjie Dental Falcon, exposes users and patients to 10 times the normal level of radiation, increasing their risks of cancer and organ damage.
The Medicines and Healthcare Regulatory Agency is asking NHS and private dentists to dispose of these devices.
It is not known how many patients may have been put at risk.
So far, 13 of the machines - sold on internet sites including eBay by a Chinese manufacturer - have been seized at a distribution centre.
At least one dental surgery has been found using the device.
Emergency testing of the product by the Health Protection Agency and scientists at King's College Hospital in London revealed that it has insufficient lead shielding inside it to protect dentists and patients from excessive radiation.
The machine's X-ray beam is also too wide, which means a patient's whole skull and brain is exposed to radiation rather than just their mouth.
And the device poses an electrical hazard because it comes with a European plug and a travel adapter that are not earthed or fused for the UK mains supply. As well as being a fire hazard, it could cause a serious electric shock (50,000 volts) to the dentist or patients.
Scientist Donald Emerton, who tested the device, said: "Over time someone operating this machine, such as a dental assistant, would be exposed to unacceptable levels of accumulated radiation and this would have an increased risk to their health.
"I certainly wouldn't want someone to use this piece of equipment to take an X-ray of me."
The MHRA believes it has shut down the UK's only distributor but says investigations are ongoing to ensure no more can be sold and used here. The problem first came to light in June 2012.
It is not yet known how many of the devices are already in circulation.
The manufacturer - Zhengzhou Tianjie Electronic Equipment Co - is currently unavailable to contact.
The Tianjie Dental Falcon was priced at about £200, a fraction of the cost of other dental hand-held X-ray sets available for sale in the UK, which can be over £4,000.
Bruce Petrie, of the MHRA's Medical Devices Enforcement Team, said: "It's vital that dentists and dental staff do not buy these dental X-ray machines from eBay or other websites because they are not approved and not safe for dentists or patients.
"We are working with eBay and other governments to ensure dentists and patients are protected."
He said anyone who had bought one of these machines should the MHRA's hotline on 020 3080 6701 or at firstname.lastname@example.org.
Barry Cockcroft, chief dental officer for England said: "It is vitally important that when buying equipment, dentists make sure it is appropriate and safe for use.
"I would urge all dental professionals to be cautious of seemingly cheap devices which may not be fit for purpose and potentially dangerous."
Richard Paynter, deputy director of the Health Protection Agency's Centre for Radiation, Chemical and Environmental Hazards, said: "We're delighted that MHRA is now taking such positive steps to ensure public and occupational protection from unnecessary radiation exposure."