(CNN) -- Olympic gold medalist Sherone Simpson of Jamaica plans to file an appeal with the Court of Arbitration for Sport (CAS) after receiving an 18-month doping suspension Tuesday, with her agent calling the ban "incredibly unjust."
Simpson, who won gold at the 2004 Olympics in the 4x100-meter relay, silver in the 100 meters at the 2008 Games and silver in the 4x100-meter relay at the 2012 Olympics, tested positive for the banned stimulant oxilofrine at last year's Jamaican national championships.
The 29-year-old revealed last year -- on the same day former 100-meter world-record holder Asafa Powell of Jamaica and former world champion Tyson Gay of the U.S. said they had tested positive for banned substances -- that she would never "intentionally take an illegal substance of any form" but Jamaica's anti-doping disciplinary panel Tuesday claimed she was "negligent," The Jamaican Gleaner newspaper reported.
The ban is backdated to last June, which means Simpson would be able to return to competition this December.
"The panel has given no written explanation as to how or why they came to this decision," Simpson's agent, Paul Doyle, said in a statement sent to CNN. "We feel that this ruling is incredibly unjust and we will be appealing to the Court of Arbitration (for) Sport immediately."
Doyle added that the case for CAS should be "very straightforward."
"Sherone took a legal supplement that was contaminated with oxilofrine," said Doyle. "Two different labs that we commissioned to test the supplement both determined that oxilofrine was present and that it was not declared on the label.
"Additionally, on our advisement (the U.S. Anti-Doping Agency) ordered the supplement directly from the company and tested it themselves and confirmed the same. Subsequently, USADA has posted a warning on their website warning athletes not to take the supplement because it contains banned substances that are not declared on the label.
"These are the core facts of Sherone's case and cannot be disputed. Typically in such a case, the athlete is given a punishment ranging from a public warning to three months of ineligibility. The fact that the panel has given 18 months suspension and have provided no explanation as to why is unacceptable in our opinion."
It was a double blow Tuesday for Jamaican athletes, since discus thrower Allison Randall received a two-year doping suspension, The Gleaner said.
Powell, who also tested positive for oxilofrine, is expected to learn his fate Thursday. Simpson and Powell share the same physical trainer, according to The Gleaner.
Simpson will be hoping for the same ruling handed out by CAS in February in the case of decorated Jamaican sprinter Veronica Campbell-Brown.
Campbell-Brown, who has won more than a dozen Olympic and world championship medals, had her two-year ban for testing positive for a banned diuretic last May overturned.