Race meetings at all British racecourses have been cancelled on Thursday due to an outbreak of equine flu.
The British Horseracing Authority (BHA) made the decision on Wednesday evening after the Animal Health Trust confirmed three positives tests from vaccinated horses in an active racing yard.
The horses from the infected yard had raced earlier in the day at Ayr and Ludlow, potentially exposing other horses across the country and in Ireland to the disease.
The BHA said in a statement that the identification of the virus in vaccinated horses presented a "cause for significant concern".
It added: "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 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."
The action to cancel Thursday's races was taken with unanimous support of the BHA's industry veterinary committee and will affect meetings at Huntingdon, Doncaster, Ffos Las and Chelmsford.
Equine influenza is a highly infectious disease of horses, mules and donkeys. Symptoms in non–immune animals include high fever, coughing and nasal discharge.
