Daryl Impey, who last year became the first African to wear the Tour de France yellow jersey, has failed a drugs test, his team Orica-GreenEdge have announced.
Impey was left off the team's squad for this year's Tour, announced on Tuesday, opening the door for 21-year-old British rider Simon Yates to take the start line in Leeds on Saturday.
On Wednesday, the Australian team revealed the reason why Impey, 29, will miss out.
"ORICA-GreenEDGE has been notified that Daryl Impey has returned an adverse analytical finding and has been notified by The South African Institute for a Drug-Free sport that he delivered a positive A and B sample for the substance Probecenid after the South African Championships on February 6, 2014," a team statement said.
"The team would like to underline that it respects Daryl Impey's right to prove his full innocence and will not comment any further until the process has run its due course and final conclusion has been made.
"As per the team's code of conduct, Daryl Impey will not feature on the team's roster until the case has been closed and he is fully eligible to ride."
Impey took the yellow jersey from team-mate Simon Gerranson on stage six of last year's Tour and wore it for two days until until Chris Froome won stage eight to Ax 3 Domaines.