Oliver Tait, Darley's chief operating officer, has resigned from the board of the Breeders' Cup because of its backflip on the use of the diuretic Lasix.
"The Breeders' Cup has reversed its previously agreed and stated position that the 2013 Breeders' Cup World Championships will be conducted without Lasix," Tait said in a statement from Darley.
"As a consequence, I have resigned from the board of the Breeders' Cup."
The anti-bleeding medication Lasix, also known as Salix, is banned as a raceday treatment in most countries but is widely used in the United States, usually about four hours before a horse competes.
The Breeders Cup board voted last week not to expand its prohibition of Salix to all races at the meeting, but opted to continue the 2012 policy of banning it in the juvenile races.