US Humane Society president and chief executive Wayne Pacelle said the decision marked the the end of the line for the foreign-owned horse slaughter industry in the United States.
"Now it's up to Congress to finish the job and protect American horses from being exported to foreign abattoirs in Canada and Mexico for human consumption overseas."
The United States Court of Appeals for the Seventh Circuit yesterday upheld the State of Illinois's decision to ban the slaughter of horses for human consumption.
Illinois is home to the last remaining horse slaughter plant in the country, at DeKalb, run by Cavel International.
In a unanimous ruling, the court rejected all of Cavel's claims and reiterated that "States have a legitimate interest in prolonging the lives of animals" and promoting the "humane treatment of our fellow animals."
Remaining appeal options for Cavel to continue its slaughter programme are now limited.
The Humane Society urged the public to back a bill before Congress that would prevent horses being transported to slaughter plants in Mexico or Canada.
"This ruling should make the people of Illinois proud to stop the last remaining horse slaughterhouse in the country," said Illinois State Representative Bob Molaro, one of the key sponsors of the bill.
"This was a hard won fight for the legislature, but the fight is not over.
"I ... hope that this decision will spur the passage of federal legislation to prevent American horses from being shipped to Mexico or Canada for butchering."
The Illinois law which banned slaughter in the state was signed by Governor Rod Blagojevich and took effect immediately on May 24.
Shortly after, Cavel International took court action seeking to block enforcement of the law.
Earlier this year, the Fifth Circuit Court of Appeals had rejected a similar effort by the horse slaughter industry to overturn Texas law banning the possession of horse meat for human consumption. In July, the federal district court in Rockford, Illinois upheld the Illinois state law, for substantially the same reasons provided by the Fifth Circuit in the Texas case, and Cavel appealed that decision to the Seventh Circuit.
"This was the final chapter in our successful efforts to close down the last remaining horse slaughterhouse in the United States," said Illinois State Senator John Cullerton, another key sponsor of the law. "We have finally stopped the slaughter of these majestic creatures."
The Animal Welfare Institue also welcomed the decision, pointing out that horses had continued to be slaughtered while the case was before the appeals court.
"We are overjoyed by the court's latest decision," said Tracy Silverman, General Counsel for the institute. "The state of Illinois did not want to be home to the nation's last remaining horse slaughter plant, and today's court ruling signals an end to this gruesome industry in the United States."
However, it warned that Cavel still had two options remaining to challenge the ruling.
It can request an en banc review where, if granted, all judges on the Seventh Circuit Court of Appeals would reconsider the decision rendered by the three-judge panel, or it can appeal directly to the Supreme Court of the United States.
Earlier this year, the Supreme Court refused to hear a similar request by the two Texas horse slaughterhouses operating in violation of Texas law. Those plants have effectively ceased operations as a result.
"The institute calls on Cavel to stop stonewalling the inevitable by prolonging this cruelty via the courts," said its deputy legislative director Chris Heyde.
"Each day it drags out this process, hundreds of horses are caught in the middle, paying the ultimate price as a result.
"The American public has made clear its desire to close this industry down," Heyde said. "That's why AWI is proud to be leading the charge to pass the urgently needed federal law to ban horse slaughter."