Soaring temperatures at Cheltenham on Thursday saw the JRL Group Mares' Handicap Chase abandoned on welfare grounds.
On an unseasonably hot afternoon in the Cotswolds, the decision was taken not to run the three-mile-two-furlong contest, which was scheduled to be the fourth race on the card.
Earlier, the Richard Hobson-trained Dame Rose collapsed and died after finishing fourth in the novice hurdle, although the British Horseracing Authority stated it was not yet known what caused her collapse, or whether it was related to heat stress.
The five-year-old was to be sent for a post-mortem to establish the circumstances of her death.
The remaining three races, one of which was a bumper, were all staged at around two miles.
Stipendiary steward Simon Cowley told Racing UK: "We've been monitoring the situation throughout the day because of the heat and we put measures in place after each race with water at the pull-up section.
"The jockeys have been telling us that a lot of the horses have been getting unexpectedly tired and we suspect this is because the horses are not acclimatised off the back of a cold and wet winter.
"So taking that in mind, we enquired into the effect of the heat on horses and with a three-mile-two race coming up, we listened to trainers and jockeys and decided a race over that far in these conditions was a little bit too risky.
"We had to take a measured view and felt it was too risky. There's a mixture of science and instinct involved but we had to look at the bigger perspective. That race in these conditions is probably asking too much.
"We looked at putting the race on the end of the card, but the temperature is not due to drop significantly."
Grand National-winning trainer Donald McCain was due to run Same Circus in the race and was not happy with the decision to abandon.
"I just think it's an appalling decision as I don't know how you can differentiate between distances," said McCain.
"If it's too hot to race, which I don't think it is by the way, then it's too hot and the whole meeting should be abandoned.
"Hopefully nothing happens in the remaining races because if it does they've given people every opportunity to give them stick.
"Is it any more testing for a staying chaser to run in a staying chase than for a juvenile filly to run over two miles at Cheltenham? I don't think it is. If they had abandoned altogether I might not have agreed, but I probably wouldn't have said anything.
"I think we should have raced."
Fellow trainer Dan Skelton did not have a runner in the race, but agreed with the decision not to run
"I didn't have a runner so it is easy for me to say, but this is unprecedented so we have to act accordingly," he said.
"I think the right decision has been made. You'll get hotter days than this in the summer, but the problem is they are coming out of a very cold winter.
"Their coats have still got a bit of growth on them, they are like seal skin in the summer, there's no hair really, so I think they are over-heating because of the time of year, more than the temperature.
"Where you draw the line is not for me to say, but I think in the circumstances, we are in April, two-mile races don't look in danger, but three-mile-twos do.
"I had a runner over that trip yesterday and while he wasn't distressed, he took a lot of cooling down and it is not a pleasant experience, but the two-mile horses have been fine. Three-mile-two is an extreme distance."
BHA media manager Robin Mounsey said: "While we appreciate that this decision may be frustrating to some, it was discussed at length with vets, jockeys, trainers and the racecourse and we hope those affected will understand that the decision was taken with welfare in mind.
"The safety and health of our participants is the number one priority for everyone involved in the sport.
"In unseasonal weather such as this, races run over longer distances may carry an increased risk of horses becoming too hot, especially when the heat has come on so quickly and horses haven't had a chance to acclimatise.
"Hot weather provisions have been put in place across the country, such as extra water and tired horses not returning to the parade ring after races, but the decision was made to cancel this one race as an extra precaution."