British horse racing was plunged into a sudden crisis on Wednesday night as it emerged that three horses in an active yard, believed to be that of Donald McCain, have tested positive for equine influenza, despite having been vaccinated against it.

All four scheduled meetings in Britain on Thursday were summarily cancelled as the British Horseracing Authority moved hastily to try to contain the outbreak. It is not immediately clear when racing will be able to resume.

Alarmingly, the BHA confirmed that the affected stable had runners at Ayr and Ludlow on Wednesday, where they were in close proximity to dozens of other horses from many racing stables. McCain is the only trainer who had runners at both meetings, suggesting that his stable, in Cheshire, is the one referred to by the BHA. Other trainers who had runners at Ayr or Ludlow include Paul Nicholls, Nicky Henderson and Gordon Elliott, who sent runners to Ayr from his base in Ireland.