Four new cases of equine flu have been discovered, potentially hampering hopes British horse racing could resume this week.
It had been thought the sport would restart on Wednesday.
But new cases were identified at the Newmarket yard of trainer Simon Crisford, the British Horseracing Authority (BHA) were confirmed on Sunday.
The outbreak of equine flu has forced the cancellation of all British racing until February 13 at the earliest, after an initial three cases were found at Donald McCain's stable in Cheshire on Wednesday February 6.
The BHA are expected to make a decision on Monday whether to resume horse racing this week.
- What is equine flu?
Equine influenza is a highly infectious disease of horses, mules and donkeys.
Symptoms in non-immune animals include high fever, coughing and nasal discharge.
It has an incubation period of between one and five days and its symptoms can persist for several weeks.
In severe cases, a horse might be unable to exercise for two or three months and a severe outbreak would have the potential to close down Britain’s racing industry for many weeks.
- When was horse racing last affected?
This shutdown echoes the similarities from the foot-and-mouth crises of 1967 and 2001.
It is not yet known how long the current shutdown of racing may have to last - but on each occasion, the racing calendar was affected for two months - and in 2001, the Cheltenham Festival was abandoned.
Thursday's cancellations come less than five weeks before the start of this year's Cheltenham Festival - the annual highlight of the National Hunt calendar.
- Can humans get equine flu?
Humans do not get infected with equine influenza.
However, people can physically carry the virus on their skin, hair, clothing and shoes, and can therefore transfer the virus to other horses.
- How have the British Horseracing Authority responded?
The governing body said "the picture is still developing and it remains the case that we will make an evidence-based decision about the situation on Monday."
It added: "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."
The action to cancel races was taken with unanimous support of the BHA's industry veterinary committee and will affect meetings at Huntingdon, Doncaster, Ffos Las and Chelmsford.
- Former jockey and current racing presenter Mick Fitzgerald explains the difficulties race horse trainers are now facing