Team Sky have pulled the Colombian rider Sergio Henao from their line-up for up to two months in order to conduct further tests after the team's regular out-of-competition tests threw up questions over his blood values.
Henao will return home to Colombia for an "altitude research programme" in an attempt to clarify the results. The 26-year-old, a key domestique for Sky, is now likely to miss the Giro d'Italia, the first Grand Tour, which begins in Belfast on 9 May.
Sky have informed the UCI, cycling's governing body, of the findings and stressed Henao has not been found guilty of any doping offence. The further tests in Colombia will be conducted by independent experts and their findings will be made public.
"Our experts had questions about Sergio's out-of-competition control tests at altitude – tests introduced this winter by the anti-doping authorities," said Sir Dave Brailsford, Sky's principal. "We need to understand these readings better. We want to do the right thing and we want to be fair. It's important not to jump to conclusions."
Brailsford added that Sky's knowledge was limited by a lack of scientific research into "altitude natives" like Henao, who was born and raised in the mountains.
Another Sky rider, Jonathan Tiernan-Locke, who won the 2012 Tour of Britain, will learn his fate in the "next month or so" after anomalies were discovered in his biological passport. UK Anti-Doping said they were close to concluding their investigation of a "difficult" case.