Newmarket-based trainer and veterinary surgeon James Tate admits the discovery of four cases of equine influenza in the town is a significant blow to the chances of racing resuming later this week.
The British Horseracing Authority announced late on Sunday evening that four vaccinated horses at the yard of trainer Simon Crisford had been found to have contracted the highly-infectious disease.
The news came after what had appeared to be a positive weekend in the fight against the virus, with no fresh cases being reported since three horses - which subsequently rose to six - were found to have virus at Donald McCain's stable in Cheshire last Wednesday.
Tate told Sky Sports Racing: "Basically this is another trace that will need to be followed and another line of investigation.
"Yesterday we were all hoping that while poor Donald McCain's yard was on lockdown, the rest of us might be able to get back going.
"Now it's Donald McCain's yard and Simon Crisford's yard on lockdown and we have to trace the Simon Crisford line of infection before we can let everybody else keep going. And then, God forbid, we get any other positives in other yards."
Whereas McCain's yard and gallops are self contained, Crisford is based in the biggest training centre in the country - meaning his horses could have come into contact with horses from other yards while working on the Newmarket Heath.
Tate added: "The virus can be transferred through the air - although only a certain amount of metres - and between staff and between horses passing in strings.
"If Simon Crisford's horses have just tested positive, none of them were infectious during the early part of last week and certainly in the latter part of last week he was exercising on the Heath in the afternoons with other at-risk horses.
"It is concerning and we just have to hope it hasn't transferred to any other yards. The long and the short of it is we were hoping to get back racing towards the end of this week and this is a setback and now that looks unlikely."
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.
Following a six-day shutdown of racing in Britain, the ruling body is due to decide on Monday evening whether racing can resume on Wednesday.