Four new cases of equine flu have been reported, potentially dashing hopes British horse racing could resume this week.
It had been thought the sport would restart on Wednesday.
The new cases were identified in vaccinated thoroughbreds at the Newmarket yard of trainer Simon Crisford, the British Horseracing Authority (BHA) said late on Sunday.
Racing has been on an enforced six-day shutdown since Thursday after an initial three cases were found at Donald McCain’s stable in Cheshire. This has risen to six cases.
In a statement, the BHA said Mr Crisford’s 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 5 February, which had been identified as a potential risk fixture.
At the moment the affected horses are all contained within this yard, it added.
A further statement will be made on Monday, with a decision on whether racing will be pushed back further than Wednesday.
The strain of equine flu which has halted UK horse racing is the same one which shut down racing in Australia for three months in 2007, affecting more than 10,000 horses.
British horse racing is worth £3.45bn, according to a 2013 report by Deloitte.
In 2001, an outbreak of foot-and-mouth disease prompted a suspension of racing for two months in Britain.
(c) Sky News 2019: Fresh blow to UK horse racing as four new cases of equine flu discovered