LONDON — Britain has canceled horse races at least until the middle of next week after three horses tested positive for equine influenza.

The British Horseracing Authority announced the decision on Thursday as races already had been halted for a day.

“The disease can take up to three days before symptoms are visible, meaning it will take until Sunday at the earliest before the B.H.A. can gather all the information required,” the authority said in a statement.

More than 100 racing yards were on lockdown on Thursday as the authorities carried out health checks, The Racing Post, a specialist publication, wrote on Twitter. Horses on these yards competed at racecourses in Ayr, Ludlow and Wolverhampton this week.