Race meetings at all British racecourses have been cancelled after three confirmed cases of equine influenza.

The British Horseracing Authority made the decision to cancel Thursday's racing after the Animal Health Trust confirmed three positives tests from vaccinated horses in an active racing yard.

The horses from the infected yard raced on Wednesday at Ayr and Ludlow, potentially exposing other horses across the country and in Ireland to the disease.

The BHA said in a statement the identification of the virus in vaccinated horses presented a "cause for significant concern".

"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.

"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."

Equine influenza is a highly infectious disease of horses, mules and donkeys. Symptoms in non-immune animals include high fever, coughing and nasal discharge.

In 2007, racing in NSW and Queensland was shut down for three months when EI spread throughout the horse population which was not vaccinated against it.

An inquiry into the outbreak found it was most likely brought to Australia by stallions imported from Japan.