Southwell abandoned a race after vets decided the weather was too hot.
The Sky Sports Racing HD Virgin 535 Novices' Hurdle, scheduled for 4.15pm, was called off when temperatures reached 28C.
Six races had already been run at the Nottinghamshire venue by the time the decision was made.
Two of them were over the same distance as the cancelled contest, of about three miles.
Clerk of the course Paul Barker said: “It has got up to 28 degrees, which was more than we expected.”
The final races of the National Hunt programme, two divisions of the Final Furlong Podcast Handicap Hurdle over two and a half miles, were held as normal.
The stewards' report said: "An enquiry was held as a result of concerns from the veterinary officer and veterinary surgeon that the weather conditions may be too hot to continue racing.
"Riders Tom Scudamore, Paddy Brennan and Sean Quinlan (jockey safety officer), trainers David Bridgwater, Sam England, the veterinary officer, a veterinary surgeon, the clerk of the course and the racecourse managing director were interviewed.
"Having considered the evidence, the stewards ordered race seven - a three mile hurdle race - to be abandoned.
"On veterinary advice, races eight and nine were deemed to have a reduced risk to horse welfare and therefore allowed to continue as scheduled."
In the race, former Flat jockey Poppy Bridgwater was due to make her debut riding over jumps on Fort Gabriel, a horse trained by her father David.
Permission was given for all horses to be dismounted, unsaddled and cooled with water on the track throughout the afternoon, before returning to the winners’ enclosure and stables.
The card was marred by a fatal injury to 2019 Cheltenham Festival winner Croco Bay.
He won the Johnny Henderson Grand Annual Challenge Cup Handicap Chase at odds of 66/1.
