Four new positive tests for equine flu have been returned in vaccinated thoroughbreds at the Newmarket yard of flat trainer Simon Crisford.
Racing is on hold until at least Wednesday while the British Horseracing Authority tests horses nationwide.
The suspension came after the discovery on Thursday of six cases of equine flu at Donald McCain’s Cheshire stable.
Crisford’s yard was named so “the Newmarket community is aware” where the infection has been found, the BHA said.
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 BHA is set to announce when racing can begin again on Monday evening.
The Animal Health Trust (AHT), which is carrying out testing on behalf of the BHA, 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.