An outbreak of equine flu has forced the cancellation of all British racing until February 13 at the earliest, after three horses tested positive for the virus, despite having been vaccinated against it.
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 severe cases, a horse might be unable to exercise for two or three months and a severe outbreak would have the potential to close down Britain’s racing industry for many weeks.
In a statement, The British Horseracing Authority (BHA) said identification of the virus in vaccinated animals presented a "cause for significant concern".
It is not yet known how long the current shutdown of racing may have to last - but Thursday's cancellations come less than five weeks before the start of this year's Cheltenham Festival in Gloucestershire - the annual highlight of the National Hunt calendar.
Race meetings at Exeter racecourse scheduled for Sunday and Monday have been cancelled.