Five race meetings have been abandoned after the Met Office issued a 'Red' extreme heat warning for Monday and Tuesday next week.
Monday's cards at Beverley and Windsor and the Tuesday fixtures at Chelmsford City, Southwell and Wolverhampton will no longer take place as the British Horseracing Authority (BHA) says it is taking "sensible precautions".
Earlier on Friday it was announced that Southwell's jumps card had been altered to start at 10am, with Arena Racing Company (ARC) also cancelling a number of greyhound fixtures next week.
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In a statement confirming the latest move, the BHA's chief operating officer Richard Wayman said: "The BHA's number one priority in the staging of any meeting is the wellbeing of all involved, human and equine.
"Following the issuing of the first-ever extreme heat warning by the Met Office, we are taking sensible precautions and have made a decision as soon as possible in order to provide certainly for those impacted."
Mark Spincer, managing director for the racing division at Arc, who are responsible for Southwell, Windsor and Wolverhampton, added: "We fully support the decision to abandon the fixtures scheduled to take place on our racecourses on Monday and Tuesday.
"We have been in close dialogue with colleagues at the BHA throughout this period of warmer weather and have been operating with a number of resources in place to assist horses and people working on our racecourses.
"Whilst we had taken steps to mitigate against the higher temperatures initially forecast for Monday and Tuesday, including moving Southwell's race times to earlier in the day, the latest updates from the Met Office and the alerts issued this morning made it clear that it was within the best interests of all concerned to abandon these fixtures.
"We will be in touch with all customers who had bookings to these race meetings shortly, and our thanks goes to the BHA team for acting swiftly in changing circumstances."