The Portuguese rider, in his first year with the American team, was set to support Alberto Contador at the upcoming Tour de France which begins on Saturday, but has since been replaced in the line-up by veteran Basque rider Haimar Zubeldia after being provisionally suspended.
Cardoso, 32, has denied any wrongdoing and requested a testing of his B sample, saying in his statement on Thursday night: "I have always conducted myself as a clean athlete."
“Today, I received notification from the UCI that my A Sample, from a urine test done at my home on June 18th, tested positive for Erythropoietin,” Cardoso wrote on his Facebook page.
“I have requested to the UCI that my B sample be tested as soon as possible.”
“Getting the chance to ride at the pinnacle of professional cycling is the greatest honor I could ever hope for, and I was looking forward to doing my best for my team and myself at the Tour,” he added.
“I believe in clean sport and have always conducted myself as a clean athlete, but I realize that this news puts a dark cloud on not just myself but also on our sport and my team, teammates and staff."
Cardoso, who rode for Cannondale before moving to Trek-Segafredo, said that he was devastated by the notification of his positive test and said he'd "seen first hand" the negative effects of doping on cycling.
“I am fully aware that I will be presumed to be guilty, but it's important to me to say that I am devastated by this news and I wanted to state that I have never taken any illegal substances," he wrote.
“I've seen firsthand through my career the awful effects that performance enhancing drugs have had on our sport, and I would never want to be a part of that. I've always tried to be a constructive influence in the peloton and on young, aspiring cyclists. It is my great hope that the B sample will come back as negative and clear me of any wrongdoing.
“Until then, I hope that those who know me, trust me when I say that I'm innocent, and that my colleagues and cycling fans everywhere don't judge me too quickly during this difficult time.”
Thank you for reading 20 articles this month* Join now for unlimited access
Enjoy your first month for just £1 / $1 / €1
*Read 5 free articles per month without a subscription
Join now for unlimited access
Try first month for just £1 / $1 / €1