Four British meetings were scheduled for Thursday - Huntingdon, Doncaster, Ffos Las and Chelmsford.
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.
All race meetings in Britain have been cancelled until next Wednesday at the earliest while the sport's governing body awaits test results from more than 100 horses which could have been exposed to equine influenza.
It is not yet known how long the current shut-down of racing may have to last - but inevitably for thousands of racing followers, and of course those directly involved in the industry, there will be uncomfortable echoes of the foot-and-mouth crises of 1967 and 2001.
Symptoms to be aware of range from an increased temperature, coughing and nasal discharge and the horse being off feed to more severe respiratory signs - your veterinary surgeon should be contacted for advice under current circumstances if these signs are present. It is rarely fatal, though it can lead to complications including pneumonia and some horses can take several weeks, or even months, to recover fully from an infection.
Horse flu is an "enzootic" disease, meaning it is native to animals, and in this case doesn't spread beyond horses and closely related species such as mules and donkeys.
Part of what's scaring people so much about this recent diagnosis of equine flu is the fact that the horses who have tested positive for the disease had been previously vaccinated against it.
The BHA warned yesterday that horses from the yard raced at Ayr and Ludlow on Wednesday, exposing other animals, including some from Irish yards to the risk of infection.
However, the statement added that runners from Britain would not be permitted to race in Ireland until further notice.
All fixtures were called off on Thursday by the BHA and one of the UK's leading trainers, Donald McCain confirmed the horses came from his Cheshire stables.
ITV Racing, which had been due to cover both Newbury's valuable meeting and significant races at Warwick this weekend, said it will instead show five races from Naas in Ireland. The BHA issued a warning to racing professionals on January 19 but, until now, no cases in Britain had been confirmed from active yards in vaccinated horses.