Horner is said to have been treated with cortisone because he was suffering from bronchitis during this year's Tour de France. Horner was handed a Therapeutic Use Exemption (TUE) by the UCI and was thus allowed to take part in this year's Vuelta a Espana, which begins tomorrow. Treatment with cortisone will reduce cortisol levels in the treated body.

However, Horner's team Lampre-Merida is a member of the Movement For Credible Cycling (MPCC). MPCC rules and regulations forbid riders to take part in races if their cortisol levels are below a certain value.

"After the necessary UCI blood tests were conducted they showed a lower cortisol level compared to the minimal level requested by the MPCC, thus the decision from the team to not allow the athlete to partake in this Vuelta even with having all the necessary UCI authorisation in order," the team said in a statement issued earlier today.

"With this decision, the team reaffirms his adherence to the principles underlying the MPCC organisation, agreeing to respect the rules and regulations and not to allow the athlete to start the Vuelta even though this being an important appointment for the athlete after an investment had been made on behalf of the team."

Horner himself is accepting the fact that he won't be able to participate in this year's Vuelta. "Of course I’m sad about this news. I was willing to try to defend the 2013 title, Vuelta a Espana was my main target of the season, the team signed my with the aim of being competitive in the Spanish race, but I accept the decision linked to the MPCC’s rules," Horner said.

"This bad bronchitis caused me a lot of problems, I’ve been suffering for it for weeks and this treatment could have allowed me to solve the problem. UCI gave authorization for the treatment, I could race according UCI rules, but my team is member of MPCC, I understand it and we all must accept this situation without regrets."

Lampre-Merida's head of medical issues Doctor Carlo Guardascione elaborated further "After the finish of the Tour de France and after the Tour of Utah where the athlete was still suffering from bronchitis, Chris Horner underwent two examinations by two specialists for his bronchitis as he had been suffering since the beginning of the Tour de France as well as during the Tour of Utah. Both specialists agreed that a treatment of cortisone by oral means was the only way to resolve this problem. All the necessary steps were taken to request a therapeutical use exemption. This authorization was given by UCI commission for the athlete to proceed with this therapy on the 15.08.2014."

"Physiologically this treatment can cause a lowering of the cortisol together with other factors such as jet lag after his travel from United States where he had a time difference of nine hours."

Cortisone has been involved in a number of doping cases, where riders have used the substance to enhance their physical performance.