It has emerged that the heavyweight boxer Tony Thompson failed a drugs test for the fight that ruined David Price’s career.

The American, who stopped Price in both February and July 2013, tested positive for hydrochlorothiazide – which is classified as a diuretic and masking agent – after their July rematch.

Price’s prospects have since diminished despite him then being considered a potential world heavyweight champion.

Following the second of the defeats the 32-year-old split with his trainers, Franny Smith and Lennox Lewis, and the promoter Frank Maloney and turned to a sports psychologist.

His latest attempts to rebuild his reputation led to a third knockout defeat, earlier this month for the European title, to Germany’s Erkan Teper.

Thompson, 43, who insisted doping should be legalised before his first defeat of Price, has fought four times since; he was banned by the British Boxing Board of Control for 18 months but that was only enforceable in the UK. The ban expired in May 2015.

Price learnt of the ban only on Sunday. An insider in his camp said the second defeat, and the way Thompson recovered from a heavy second-round knockdown, caused Price to “seriously question himself – he was low, he thought his career was over”, and contributed to his decline.

UK Anti-Doping said in a statement the delay in Thompson’s failed drugs test being revealed was a consequence of the American’s response to the charge. “The athlete instigated a lengthy legal process, including filing an appeal which he failed to progress and which was ultimately dismissed,” the statement read.

The decision was published by the National Anti-Doping Panel. It revealed Thompson argued the banned substance entered his body through a medication taken for high blood pressure – not to control his weight or hide other substances – but he did not provide sufficient proof from a medical professional to support this.

He also claimed to be unaware hydrochlorothiazide was banned. He was given a separate 12-month ban by the Austrian Boxing Federation for the presence of the same substance when fighting Kubrat Pulev in August 2013, three weeks after UKAD contacted him about their case.

Thompson continues to maintain his innocence. When contacted yesterday he said: “I did fail a drugs test but everybody knows what I failed it for – high blood pressure medicine.

“How could it be wrongdoing? I’m taking it for what the doctor prescribed it for. I have high blood pressure, I have for the last 15 years.”