All British horse racing was cancelled on Thursday after three animals tested positive for equine influenza despite having been vaccinated against the disease.
Horses from the infected yard raced on Wednesday at Ayr and Ludlow, potentially exposing a significant number of animals from yards across Britain and in Ireland.
Four meetings were scheduled for Thursday -- at Huntingdon, Doncaster, Ffos Las and Chelmsford.
"The British Horseracing Authority (BHA), with unanimous support of the BHA's industry veterinary committee, has taken the decision to cancel racing at all British racecourses on Thursday 7 February 2019," it said in a statement.
"This is following the BHA being informed this evening by the Animal Health Trust of three confirmed equine influenza positives from vaccinated horses in an active racing yard."
Equine influenza is a highly infectious disease of horses with symptoms including high fever, coughing and nasal discharge.
The outbreak at the infected yard follows the identification of a number of cases across Europe and the UK, including several in vaccinated horses, the BHA statement said.