The second Classic of the season was delayed for nearly 30 minutes after the filly, owned by Raymond Mould, tried to escape beneath the starting stalls and became stuck.

Gray Pearl could not be moved for some considerable time, despite the best efforts of the stalls handlers, and was taken away in a horse ambulance.

A decision was then taken by racecourse vet Simon Waterhouse and Chris Hammond, the attending BHA veterinary officer, to put her down.

In a statement Hills said: “I would like to place on record my thanks to all those who helped in the treatment of Gray Pearl.

“The pressure and tension at the start of a Classic, particularly one involving 18 fillies, is significant and yet everyone involved, from the stalls handlers to the vets and the starters, remained completely calm and professional throughout.

“In all my riding career I have never experienced an incident such as that which happened on Sunday. One moment the filly was standing quietly in the stalls and then all of a sudden she dived down and then did not, or could not, get back up.

“The response from the starting team was immediate and I would like to pay particular thanks to the stalls handlers who not only looked after the filly, but also regularly checked on myself, as well as doing the not inconsiderable job of unloading the other runners and looking after them while the vets examined and treated Gray Pearl.

“Their priority at all times was the welfare of Gray Pearl. Everyone involved did all that they could to help her and then to minimise her distress as soon as the gravity of the injury became apparent.

“The filly had got herself into such an awkward position and it was a major operation to move both sets of stalls so that she could receive the best treatment possible in very difficult circumstances.

“Everyone connected to the horse is very upset and sad to have lost her. But the quality of care and professionalism shown by those who attended to her is some consolation and we are very grateful to all concerned.”