The International Equestrian Federation (FEI) decided in September that both Britain and Belgium would be relegated to the promotional league following a tie for eighth place in the Nations Cup super league competition last year.
But the BEF appealed that decision, arguing that Britain’s show jumping team should remain in the top tier.
Their argument was that under the existing rules Team GB was initially declared eighth and should not have been relegated with Italy (10th place) and Belgium.
But the FEI took the opportunity to cut the number of teams in the competition and changed the rules.
A hearing of the FEI disciplinary tribunal took place in Switzerland in December but announced it would not intervene in the September decision — and that Britain and Belgium remain relegated.
Now the CAS must make the final decision.