An outbreak of equine flu has forced the cancellation of all British racing until February 13 at the earliest, after three horses tested positive for the virus, despite having been vaccinated against it.
Donald McCain has confirmed three horses in his yard have tested positive for flu, but is unsure as to how they contracted the virus.
A spokesperson from the BHA said: "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".
"The BHA has worked quickly to identify which yards could have potentially been exposed today and identify the further actions required".
The decision that British horses should be barred from entering Irish races was deemed "a prudent step in the short term" by the IHRB's Dr Lynn Hillyer as the risk to disease in Irish thoroughbreds was unchanged.
A decision will be made on Monday.
Ludlow is not due to host another meeting for another fortnight, but Wolverhampton Racecourse has cancelled Saturday's fixture and is offering full refunds or transfers for everybody who pre-booked.
Racing has not been hit in such circumstances since 2001, when the foot and mouth outbreak saw the Cheltenham Festival called off.
The BHA has said it is far too early to say if the showcase meetings at the Cheltenham Festival and Aintree will be affected.
Equine flu is a highly contagious viral disease which can affect all equine species. With McCain-trained horses running in Britain this week that potentially exposed a significant number of horses from yards across Britain and in Ireland.
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.
All roads lead to Cheltenham for racing's participants and punters but there is every hope the year's most important meeting will go ahead next month given that the mass vaccination already sanctioned by the sport should ensure the action on the Turf resumes before then.
A further update on the possible continued extent of disruption is expected from the BHA - with a packed weekend of Cheltenham trials and other big races scheduled at Newbury, Warwick, Musselburgh and in Ireland.