The British Horseracing Authority will make a decision on Monday evening whether racing can resume on Wednesday following an outbreak of equine flu.
All racing has been off since Thursday while the BHA tests horses nationwide.
Four new positive tests were returned in vaccinated thoroughbreds at the Newmarket yard of flat trainer Simon Crisford on Sunday.
The BHA said the decision will be released after 22:30 GMT on Monday to weigh up the most recent evidence.
It will take into account data already gathered and the results of tests being carried out by the Animal Health Trust (AHT) on Monday.
Racing was suspended after the discovery on Thursday of six cases of equine flu at Donald McCain’s Cheshire stable.
The AHT is prioritising testing horses that could have been exposed to horses that have tested positive or horses from yards that have returned positive tests.
If racing does return on Wednesday, the BHA said declarations for any fixtures stages will be made at 10:00 GMT on Tuesday.
Crisford’s yard was named so “the Newmarket community is aware” where the infection has been found, the BHA said.
But Crisford says there is “no obvious connection” between the horses that have tested positive for equine flu at his yard and their stablemate who ran at a potential risk fixture last week.
Crisford confirmed Sajanjl, who ran at Newcastle last week, has tested negative.
In a statement, he said none of the four horses who tested positive displayed any clinical signs of respiratory illness prior to the mandatory swabbing undertaken last Friday.
“All horses at Kremlin House Stables, totalling 92 boxes, undergo a strict vaccination check and programme on their arrival,” the statement added.
“All four identified horses have been vaccinated within the last six months along with the rest of the yard and in line with vaccination protocol.”
The yard is one of the 174 to be tested because runners from the stable competed at the fixture at Newcastle on 5 February, which had been identified as a potential risk fixture.
All the affected horses are contained within Crisford’s yard, the BHA said.
The AHT is working through “several thousands of samples” received from yards across Britain.
Earlier on Sunday, the BHA reported that around 1,500 samples had been analysed without a positive test.
The authority also confirmed that testing had been carried out on the remaining 27 horses from the yard of Rebecca Menzies – where testing of three suspected cases came back negative on Saturday.
All horses in the yard have tested negative, but it will “remain under close surveillance and further testing will be carried out”, the authority said.
Equine flu – not unlike human flu – is endemic in Britain, where racehorses are vaccinated against it. The virus is generally not thought to be life-threatening, but limits the competitive capability of horses.
An unvaccinated non-thoroughbred horse was put down in Suffolk after developing complications following an outbreak of equine influenza.
In a separate case, 10 unraced two-year-old thoroughbreds in the same county were found to have contracted the highly contagious virus.
There have been outbreaks of equine influenza in nine English counties since the start of 2019.
One case involves a vaccinated non-thoroughbred horse in stables at a fee-paying school in the south west of England.
Cornelius Lysaght, BBC horse racing correspondent
Just when people were thinking that the light at the end of the tunnel was getting brighter…
After two days of no positive test among hundreds of swabs studied by scientists, the feeling was that the resumption of racing this week, maybe on Wednesday, was very much on the cards.
But this would appear to jeopardise that hope, for flat racing on the all-weather tracks anyway.
The Crisford operation at Newmarket is flat racing only, and there’s no obvious connection with the McCain string – certainly the two stables have had no runners at the same venues of late.