Hundreds of dental patients have been offered blood tests over fears they could have been exposed to HIV or Hepatitis B or C.
Dirty equipment may have been used to treat 563 patients of a former hygienist at [email protected]
Public Health England (PHE) said it was investigating "potential breaches of infection control procedures".
The practice said an independently contracted hygienist had been dismissed following an investigation.
Letters have been sent to all the Hertfordshire practice's patients who are understood to have received dental ultrasonic scaling from the self-employed hygienist, Ekta Parikh.
Principal dentist Dr Vishaal Shah said: "As soon as we became aware of behaviour from a hygienist that did not meet the rigorous standards we expect from our entire team, we immediately launched a full investigation, and escalated the matter to all key authorities.
"We understand anyone receiving these letters will have concerns and while there is an extremely low risk of exposure, all these patients are being offered detailed information, and a dedicated number to call to book an appointment for blood testing as a precaution and get further advice."
A PHE statement said equipment used by Ms Parikh "was potentially not cleaned adequately" but the risk to patients was "extremely low".
Dr Jorg Hoffmann added: "Testing is being offered as a precautionary measure. We know patients will be anxious about this situation and they will be supported by the NHS and PHE throughout.
"Effective treatments are available for all blood-borne viruses, which is why it was important to identify anyone who may have been put at risk of infection so testing and treatment can be offered."
In February the General Dental Council ruled that Ms Parikh was still allowed to practice, but imposed conditions for the next 18 months while it carried out its investigation into the hygiene breach.