Horse racing at Doncaster Racecourse has been cancelled today due to an outbreak of equine flu.
It is one of four meetings across the UK scheduled today that have been cancelled by the British Horseracing Authority.
The decision comes after three vaccinated horses in an active yard tested positive for the disease.
Horses from the infected yard raced on Wednesday, potentially exposing a significant number of horses.
Equine flu is a highly contagious respiratory virus which affects horses, mules and donkeys.
Symptoms in non-immune animals include high fever, coughing and nasal discharge.
Four British meetings were scheduled for Thursday - Huntingdon, Doncaster, Ffos Las, Chelmsford.
"The fact that the cases have been identified in vaccinated horses presents a cause for significant concern over welfare and the potential spread of the disease and the action to cancel racing has been viewed as necessary in order to restrict, as far as possible, the risk of further spread of the disease," the BHA said in a statement.
"The BHA has worked quickly to identify which yards could have potentially been exposed today and identify the further actions required.
"The BHA is presently communicating with yards potentially exposed to ensure appropriate quarantine and biosecurity measures are put in place and horse movements restricted to avoid possible further spread of the disease.
"The full extent of potential exposure is unknown and we are working quickly to understand as much as we can to assist our decision making."