The murky business of doping in sport went under the microscope on Wednesday when the trial of a German doctor accused of masterminding an international blood-doping network dismantled last year opened in Munich.

Sports physician Mark Schmidt, 42, from Erfurt, and four co-defendants who allegedly aided him, are accused of helping at least two dozen athletes undergo blood transfusions to boost performance illicitly from "the end of 2011 at the latest", according to state prosecutors.

"One must not forget, this is a business with a hard currency -- money," sports lawyer Michael Lehner said ahead of the trial.  

"We are not dealing with an athlete who dopes himself, but with the business of doping."

So far, 23 skiers and cyclists from eight countries are known to be involved.

The most prominent are former cyclists Alessandro Petacchi, a stage winner of the Tour de France, and Danilo Hondo, who won two stages of the 2001 Giro d'Italia, but more famous names could emerge.

"More can -- and should -- come out than what we already know," Lehner added.

"There are certainly more athletes involved, the network will have been bigger."

The case echoes that of sports doctor Michele Ferrari, who worked with disgraced cyclist Lance Armstrong and was given a 12-month suspended jail sentence by an Italian court in 2004 for malpractice.

If found guilty in Munich, Schmidt and his co-defendants could be jailed for up to ten years under Germany's Anti-Doping Act introduced in 2015.

Schmidt has already been held in custody for 16 months.

Caught red-handed 

The sports doctor was one of several arrested in Germany as part of Operation "Aderlass", "blood letting" in German, which involved raids at the Nordic world skiing championships in Seefeld, Austria in February 2019.

Two hours before the start of the men's 15km cross-country event, five athletes and two suspects were detained on site.

One Austrian athlete was caught during a blood transfusion.

During the course of the investigation, senior prosecutor Kai Graeber said a "terrific" amount of evidence was uncovered.

Around 30 witnesses are expected to testify with a verdict expected before the end of December.

Blood doping is aimed at boosting the number of red blood cells, which allows the body to transport more oxygen to muscles, thereby increasing stamina and performance.

Schmidt is alleged to have helped skiers who competed at both the 2014 and 2018 Winter Olympics and cyclists who raced at the 2016 summer Olympics, as well as the Tour de France, the Giro d'Italia and the Vuelta a Espana.

As part of Aderlass, Austrian cyclist Georg Preidler was given a 12-month suspended sentence for sports fraud by a court in Innsbruck in July.

Among others, Austrian cross-country skier Johannes Duerr, whose doping confessions in a documentary aired by German broadcaster ARD helped trigger Aderlass, was sentenced to a 15-month suspended sentence for doping in January.