Kid Galahad's two-year boxing ban will stand after the UK Anti-Doping agency rejected his appeal.
Sheffield's unbeaten super-bantamweight was the European and Commonwealth champion when he tested positive for a banned steroid after beating Adeilson Dos Santos in September last year.
Galahad argued he drank a protein shake which was spiked by his brother, who backed up the claim with a sworn affidavit.
But UKAD's appeals panel were not convinced by the brother's admission and concluded that "on the evidence before this tribunal the athlete has not established on the balance of probabilities how the prohibited substance entered his body."
Galahad's ban is backdated to September 20, the date of the Dos Santos fight, which will now go down on his record as a disqualification.
UKAD chief executive Nicole Sapstead said: "Kid Galahad is one of the country’s leading boxing talents and through his own actions he has jeopardised his promising career, and his reputation.
"All athletes, at all levels, need to understand the importance of Strict Liability. They are solely responsible for any banned substance that is found in their system, regardless of how it got there or whether there was an intention to cheat or not.
"The principle of Strict Liability can be challenging for an athlete. They have to ensure that they understand the anti-doping rules and that their family, friends, coaches and athlete support personnel understand them too.
"They need to be aware of the risks their career faces if they test positive, and manage that risk at all times."