The decision has been made by the British Horseracing Authority after three vaccinated horses in an active yard were tested positive for the disease on Wednesday.
Newbury's "Super Saturday" meeting - where last year's Cheltenham Gold Cup victor Native River was due to have a final race before his title defence - is among the casualties, although there are hopes some of the big races may be rescheduled for a later date.
"The signs will probably start with what we call poor performance syndrome - a horse might be off its food, and then there might be the development of a fever, nasal discharge and coughing".
We have an industry veterinary committee meeting scheduled for later on Thursday evening where we'll be briefed by the current state of play by the Animal Health Trust. An outbreak in New South Wales in 2007 resulted in the closure of Sydney's racetracks from late August until early December, although this was in an unvaccinated racehorse population.
In a statement, the BHA said identification of the virus in vaccinated animals presented a "cause for significant concern".
The British Horseracing Authority released a statement on Wednesday, revealing a stable affected with equine flu had runners at Ayr and Ludlow earlier that day.
The BHA informed the industry that it is taking immediate action over the positive tests, as there is potential for the disease to spread, reports GloucestershireLive.
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.
Donald McCain's three infected horses had all been vaccinated against equine influenza.
It is thought that more horses at race yards are likely to be infected with the disease, which takes 48 hours to incubate, and three days for tests to come back.
"It can be quite serious, especially for a race horse, which might take six months to fully recover and return to racing", says the chief vet at Uttoxeter and Aintree racecourses, who is also chairman of Fyrnwy Equine Clinic at Baschurch in Shropshire.
"The worst part of it is that we are starting to miss races that were part of the horses" preparation, ' he added. With McCain-trained horses running in Britain this week that potentially exposed a significant number of horses from yards across Britain and in Ireland. "We have three confirmed cases and this morning have taken blood and swabs from all the others for testing".
"This precautionary approach is meant to ensure we put the health of the horse population and control of the virus first, and avoid any unnecessary risk that might come from returning to racing too quickly".
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