Probenecid is used to mask the use of doping substances, anabolic drugs in particular.
Impey was not included in Orica-GreenEdge's nine-man Tour de France team, which came as a surprise when the Australian outfit made their announcement on Tuesday.
"Further to the announcement of the Tour de France team and in the interests of full disclosure, I confirm that on 23 June 2014, I was notified by William Newman, the President of Cycling South Africa, about an adverse analytical finding for Probenecid following an in-competition test at the national time trial championships on 6 February 2014," Impey said in a statement.
"I had no knowledge of Probenecid nor have I ever taken the substance knowingly in any manner. I am committed to drug-free sport and fully intend to take all steps necessary to clear myself of any suspicion.
"The notification came as a complete shock to me just days before the start of the Tour de France, particularly since anti-doping tests conducted on me on 8 and 9 February 2014 yielded no adverse results.
"I immediately flew back to South Africa and was present at the analysis of my B sample on 27 June 2014, the results of which confirmed the initial analysis."
The Tour de France starts in Leeds, England on Saturday.
Last year, all-rounder Impey became the first African rider to wear the yellow jersey on the Tour de France when he led the overall standings during stages 6 and 7.
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