Christiansen was due to appear at her fifth Paralympics, having debuted at Athens 2004.

She won gold in the individual freestyle test grade Ia at the last three Paralympics, the mixed team event at Beijing 2008 and the championship test grade Ia and team championship events at both London 2012 and Rio 2016.

However, the 33-year-old will be absent from Japanese capital due to the issue with her horse Innuendo III, whose stable name is Louie, which Christiansen said meant he "wouldn’t have been able to give his best."

In a post on Instagram, Christiansen said she was "heartbroken" that she will not take part, and "truly gutted" for those in her team and everyone who had helped to fundraise and donate to allow her to take part at the Paralympics, but is looking forward to Paris 2024.

Christiansen also wished her replacement Georgia Wilson well for Tokyo 2020.

Wilson won a freestyle gold at the 2019 European Championships riding Midnight on top of an individual and team silver, and will make her Paralympic debut.

Natasha Baker, Sir Lee Pearson and Sophie Wells are also part of Britain’s Para equestrian team for the Games.

Britain’s Chef de Mission Penny Briscoe offered her sympathy to Christiansen.

"This is clearly desperately disappointing news for Sophie who is an incredible athlete, ambassador and advocate for the Paralympic movement," Briscoe commented.

"She has set the standard at recent Games and we were looking forward to seeing her compete in Tokyo.

"An injury to either rider or horse so close to the Games is very unfortunate and we will work with British Equestrian to ensure Sophie has the support she needs.

"Georgia’s selection clearly demonstrates the strength in depth in British Para Dressage and we are excited to see her make her Paralympics debut in Tokyo."

Christiansen also added that she hopes Para equestrian will be granted more support in the build-up to Paris 2024.

"I hope that the spotlight on the Paralympics will help showcase the standard of horses and coaching we now need in Great Britain, especially for the lower grades," she said.

"Finding the right horse this cycle has been incredibly tough, and in order to get back on top and also help the next generation, we need owners, horse scouts and better coach training.

"I hope that the equestrian community will feel informed enough that they can support Para dressage riders more, just as they do for the able-bodied teams."

The Tokyo 2020 Paralympic Games are due to start on August 24 and end on September 5, with equestrian events being held from August 26 to 30.