Z�ck, 40, who holds the second-fastest Ironman time in history, tested positive for etiocholanone - a metabolite of testosterone.

In a statement on his website, he said: "I have never taken banned substances to improve my performance."

He said he had used supplements in training, all of which were labelled as containing no prohibited substances.

Z�ck had been expected to make his last professional appearance on Sunday at the Ironman Germany event in Frankfurt.

However, he pulled out a week before, citing back problems and declared his career over.