The news comes after three horses stabled at Donald McCain's yard in Yorkshire tested positive for equine flu despite being vaccinated.
The BHA has since confirmed six horses from McCain's yard have now returned positive tests for equine flu.
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.
In a statement, The British Horseracing Authority (BHA) said identification of the virus in vaccinated animals presented a "cause for significant concern".
"We, along with the whole racing industry, will be guided by the BHA as to the next steps", he said.
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.
Earlier, on ITV's Good Morning Britain, the BHA's chief executive Nick Rust said he was confident the authority's quick response and a "war-room" of vets would restrict the shutdown in racing to a few weeks rather than for months.
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A decision on whether racing can resume next Wednesday will be made by the BHA on Monday.
Racing has not been hit in such circumstances since 2001, when the foot and mouth outbreak saw the Cheltenham Festival called off.
Erring on the side of caution isn't the most popular move but the decision to immediately cancel racing at Huntingdon, Doncaster, Ffos Las and Chelmsford yesterday is to be admired in an attempt to the nip the problem in the bud.
"When I first heard the news, like everyone else, my thoughts turned to the Cheltenham Festival, our sport's FA Cup Final, and how terrible it would be for so many people if it was put in jeopardy", ITV racing presenter Ed Chamberlin told Reuters.
The Grand National, the world's greatest steeplechase, is due to take place in Liverpool on April 5 and could also be under threat if the disease is not brought under control.
"It added that a plan to reschedule this weekend's key races will be constructed".