Racing was called off at four venues on Thursday - Ffos Las, Huntingdon, Doncaster and Chelmsford - after three horses from Donald McCain's yard in Cheshire tested positive for the disease.
But it added that "at least three more days are required before it will be possible to make a decision about whether it is safe to resume racing". The incubation period is usually days but recovery can take weeks, or even months.
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.More news: New Skype update introduces background blur for video
All horse racing in Britain has been cancelled until at least Wednesday, February 13.
Horses from the infected stable, which has not been named, raced on Wednesday at Ayr and Ludlow, which raised the possibility that a number of horses in their vicinity could have been exposed to the highly-contagious infection.
"The BHA were contacted immediately and we are liaising closely with them about bio-security and management of all the horses at Bankhouse".
Irish trainers Gordon Elliott, John Carr, Shane Nolan, Stuart Crawford, Ronan McNally, Caroline McCaldin and David Christie have had the runners they had in Britain on Wednesday quarantined, away from their stable.More news: Bob Stoops Named Head Coach of Dallas XFL Franchise
"It was by following this protocol that the positive results for equine flu came to light yesterday evening". Horses remain infectious for about a week. The fact that the cases have been identified in vaccinated horses presents a cause for significant concern over welfare. This could have spread the disease to other racing horses, with risks of exposure to the rest of the United Kingdom and Ireland.
The strain is most common in animals, however, and includes both swine and bird flu.
Can humans catch equine flu?
Although symptoms can be mild in vaccinated horses, the virus can be very unsafe to unvaccinated horses, although it is rarely fatal. Is horse flu unsafe to humans?More news: Sixers trade Markelle Fultz to Magic
The measure taken by the BHA are not just to protect racehorses, but Britain's wider equine community, as it is not compulsory for all horses to be vaccinated.