Asafa Powell has described his 18-month doping ban as "unfair and unjust" and confirmed his intention to appeal to the court of arbitration for sport.

The sprinter was banned on Thursday by a disciplinary panel of the Jamaican Anti-Doping Commission following his positive test for the stimulant oxilofrine at last summer's Jamaica national championships. The ban is backdated to the time of the offence, meaning Powell will be eligible to compete again in December.

The ruling on the 31-year-old comes two days after his training partner Sherone Simpson was also given an 18-month ban after testing positive for the same stimulant at the national championships. Powell, who held the 100m world record between 2005 and 2008, said in a statement: "This ruling is not only unfair, it is patently unjust. Panels such as these, I understood, were assembled to allow athletes who consciously or unconsciously come into conflict with the rules of sport a chance at equitable redemption. Unfortunately, this was not the case."

Powell claimed that he took a legal supplement, Epiphany D1, which was contaminated with oxilofrine. His statement continued: "My team commissioned two private laboratories that confirmed that oxilofrine was present in the supplements, despite it not being listed as an ingredient on the bottle nor on its website.

"I would also like to share that, upon realising that the supplement contained oxilofrine, my team made contact with both the World Anti-Doping Agency (Wada) and the United States Anti-Doping Agency (Usada), who not only ordered samples of the supplement from the manufacturer, but also tested and confirmed our findings.

"The Usada has also since issued a warning on their website for athletes to avoid the product as it contains banned substances. This is the first time in nearly 12 years of being in the sport and over 150 tests that I have had an adverse finding. It is for a stimulant, a stimulant that is only banned during competition and experts have declared has no performance-enhancing effects.

"Sanctions for a stimulant and this kind of infraction usually range from public warnings to a ban of three months, six months in the most extreme cases. I was and am still more than prepared to accept a sanction that is in line with the offence. Instead, nine months later, what has been handed down is clearly not based on the offence nor the facts surrounding it. My team has begun preparations for an appeal to the court of arbitration for sport in Switzerland.

"I want to reiterate that I have never knowingly taken any banned substances. I did all the necessary checks before taking Epiphany D1 and it is my hope that the CAS will prove to be a more open and fair avenue for the review of all the facts in my case – facts and truth that were not taken into consideration at my initial hearing."

If upheld, the ruling means Powell will miss this summer's Commonwealth Games in Glasgow. He won the 100m gold medal at the 2006 Games in Melbourne and has been a gold medallist at the Olympics and world championships in the sprint relay.

But Great Britain's former marathon world record holder Paula Radcliffe believes it is always the athlete's responsibility. "He should really run everything through the system before he takes it," she said. "Whatever the length of the ban I think the fact a high-profile athlete's got caught because of supplements should send a warning out. It should send out a warning to people not to cheat that you can get banned for four years but it doesn't always work, does it?

"We're not a sport that's saying: 'OK we're just going to sweep the problem under the carpet and not bother trying to improve it the testing or trying to catch people.' We are doing that so you are going to then pay the price in terms of the sport taking a media hit when you do catch people, but at least you know the sport is trying to make itself a clean sport."