An outbreak of equine flu forced racing into a dramatic six-day shutdown to blow the biggest hole in the sport's programme since the foot-and-mouth crisis that put paid to the Cheltenham Festival in 2001.
The decision comes after three vaccinated horses in an active yard tested positive for the disease.
Ayr and Ludlow confirmed the BHA had ordered a "level 2 deep clean" which requires the racecourses to disinfect all facilities where horses were prepared pre- and post-racing.
Horses from that stable were in action at Ayr in Scotland and at Ludlow in Shropshire on Wednesday, as well as Wolverhampton on Monday, potentially exposing 100s of horses from Britain and Ireland.
Leading Irish trainer Gordon Elliott had horses running at Ayr and has confirmed in his blog with Betfair.com that the horses in question are now quarantined in an isolation yard which is a 25-minute journey from his training base in Meath.
Equine influenza is a highly infectious disease of horses, mules and donkeys occurring globally caused by strains of Influenza A virus.
The BHA issued a statement on Wednesday night, saying that 'The fact that the cases have been identified in vaccinated horses presents a cause for significant concern over welfare'.
"Symptoms may include a raised temperature, cough and nasal discharge".
Donald McCain's yard was the only one represented at both tracks.
The Animal Health Trust are working alongside the racing industry on the matter and a further update is expected later on Thursday.
In a statement, 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". "This may then allow declarations to take place on Tuesday in time for racing on Wednesday". It follows we would never race any horses that we could have known were infected.
"When new horses arrive at our yard we try as much as possible to keep them separate, but at this stage can not know if the infection came from recent arrivals or from horses returning from racing".
Racing was, however, set to go ahead in Ireland at Thurles on Thursday.
A BHA statement read: "The British Horseracing Authority (BHA) has this afternoon taken the decision that racing will not resume in Britain until Wednesday February 13 at the earliest, including fixtures programmed by the Point-to-Point Authority".