Britain's double Olympic dressage champion Charlotte Dujardin says she is "truly devastated" after being eliminated from the European Championships.
And the 34-year-old has also referred to "those who have taken to social media in the cruellest of ways" in an emotional message she posted on Tuesday night.
Dujardin and her new major competition ride – 10-year-old mare Mount St John Freestyle – looked to have anchored Britain into team silver medal position following a score of 81.910 per cent.
But delight turned to despair in Rotterdam when a small trace of blood was found on the horse's left flank following a routine post-competition check by stewards.
Dujardin is eliminated from the whole championship, meaning she cannot contest individual medals in Thursday's grand prix special, which is also a qualifier for the showpiece individual freestyle on Saturday.
Equestrian sport's world governing body the Federation Equestre Internatonale said: "Blood was found on the left flank of the horse in the post-competition check. In accordance with article 418.104.22.168 of the FEI dressage rules, this results in elimination.
"Elimination under this rule does not imply there was any intent to injure the horse, but the rules are in place to protect the welfare of all competing horses."
Dujardin wrote on Facebook: "Today has been one of the most difficult days of my career, and I'm finding this really hard here in Rotterdam knowing what to write, to you, my supporters, as I am truly devastated.
"Social media has always been something I have embraced and something I have utilised to share my journey with horses with you all. Today, it has been a very different place for me.
"Freestyle, like all the horses in my life, have become my family, and all the moments that keep the team and myself in this unconditional love affair are the reasons we get up every day and do what we do.
"The friendship, the partnerships and the priceless moments that make us smile far outweigh all the success, and I have always felt the privilege of being with these horses that give me so much. Today was a sadness I have not experienced before.
"I am sorry if I have let you down. I'm sorry to have let the team down.
"They say the challenges in life make us stronger, and whilst I am heartbroken as I write this, please know I am grateful to all of you who have sent your well wishes.
"To those who have taken to social media in the cruellest of ways today, each and every one of you is entitled to your opinion and I cannot stop that.
"I can only thank those of you who believe in me – my team, the Blundell family (owners of the horse), all of who have gone above and beyond today.
"And lastly to the lovely Freestyle, for giving me such a beautiful ride in the arena and making me as proud as she always does."
Dujardin's elimination meant that the London and Rio Olympic champion had her score erased from the team event and Britain's overall total dropped by almost eight points.
The combined efforts of Carl Hester, Gareth Hughes and Charlotte Fry – Hester and Hughes scored grand prix personal bests – left Britain on 228.991, which was only enough for fourth behind winners Germany, silver medallists Holland and third-placed Sweden.
The British Equestrian Federation said it would not be appealing against the decision, and described it as "fair".
In a statement, the BEF said: "Following their test, a routine post-test inspection by the FEI stewards revealed a small trace of blood on the flank of Mount St John Freestyle, the ride of Charlotte Dujardin.
"As is the FEI rules, any blood found during this inspection results in elimination.
"Having consulted with our dressage team veterinarian Andre Buthe, who also inspected the horse, we feel this decision is fair in respect that the welfare of our horses and riders is absolutely paramount. We will not be appealing their decision."