The British Horseracing Authority has announced four cases of equine influenza have been identified in vaccinated thoroughbreds at the Newmarket yard of trainer Simon Crisford.
The findings are a blow to racing's hopes of resuming action on Wednesday, following an enforced six-day shutdown following the three cases - which subsequently rose to six - of the highly-infectious disease found at Donald McCain's stable in Cheshire.
The BHA said on Sunday night that Crisford - former long-standing racing manager to Godolphin - had been named "in order to ensure the Newmarket community is aware of the yard at which an infection has been identified".
It added: "No non-urgent journeys should be made to this yard - including by the media - and anyone who wishes to travel to the yard is advised to first contact the trainer's office.
"This yard is one of the 174 which has been required to undergo testing due to the fact that runners from the stable competed at the fixture at Newcastle on February 5, which had been identified as a potential risk fixture.
"At the moment the affected horses are all contained within this yard.
"The Newmarket community - including licensed trainers, veterinary surgeries, farriers, racing school and all other relevant bodies - are being advised to continue to show increased vigilance in biosecurity."
A statement from the BHA added: "A further update will be issued as early as possible tomorrow.
"This will include further details of action that is being taken off the back of this incident, and how the process will operate for the decision which will be made tomorrow evening regarding whether it is possible to resume racing on Wednesday, February 13."
The news comes after what had appeared to be a positive weekend in the fight against the disease, with no fresh cases being reported either on Saturday or up to a previous BHA statement issued shortly after 4.30pm on Sunday.
Some 720 tests had returned negative on Saturday and around the same total was estimated in addition to that on Sunday, from over 2,000 received by the Animal Health Trust in Newmarket.
Speaking in the afternoon update, David Sykes, director of equine health and welfare for the BHA, had said: "The data is encouraging and provides a further indication that the precautionary safety measures have helped to contain the spread of disease.
"However, the picture is still developing and it remains the case that we will make an evidence-based decision about the situation on Monday.
"It remains paramount that, for the sake of our horse population, we do not take any unnecessary risks. This is not a common cold, it is a highly contagious and potentially serious disease."
Racing in Britain was cancelled on Thursday, following an announcement by the BHA on Wednesday night of the McCain outbreaks, with the yard having had runners earlier in the week.
However, meetings are continuing to take place as scheduled in Ireland, albeit with British runners prevented from travelling.