Cyclist Daryl Impey, who became the first South African to wear the yellow jersey in Tour de France history last year, has failed a drugs test.
Impey tested positive for the banned substance Probenecid at the South African championships in February.
In a statement on his personal website, Impey said: "I am extremely distressed by the finding, as I have always raced clean throughout my career."
The 29-year-old Orica-GreenEdge rider has been suspended by his team.
He added: "I remain confident I will be vindicated and proved innocent of any wrongdoing. Every result I have achieved to date has been because of hard work and dedication on my part."
|Daryl Impey factfile|
|Born Johannesburg, December, 1984|
|Played football before taking up cycling in 1997 with his brother and dad|
|Signed his first professional contract in 2003 for the Microsoft Cycling Team|
|His best win to date has been the 2009 Tour of Turkey|
|Described wearing the yellow jersey at the 2013 Tour as "a dream realised"|
Impey, a former team-mate of Britain's defending Tour De France champion Chris Froome, was left out of Orica-GreenEdge's squad for this year's Tour.
His place was taken by 21-year-old British rider Simon Yates, who will take to the start line in Leeds on Saturday.
Impey won the yellow jersey, worn by the overall leader, on stage six of last year's Tour - wearing it for two days.
Froome said he has sent a message to Impey - a close friend of his - and that he hopes "things get resolved".
He told BBC Sport: "I haven't spoken to Daryl, but it's shocking news to see that he's returned a positive test. Especially when it is someone you know personally, it makes it a lot more difficult.
"The same can be said about Roman Kreuziger - right on the eve of the Tour de France.
"It obviously doesn't paint a very good picture of our sport and it really is a shame."