Alessandro Petacchi, one of cycling's most successful sprinters, is among four riders to be provisionally suspended by the sport's governing body, the UCI, for alleged involvement in an Austrian-German doping ring.

The 45-year-old Italian, who retired in 2015 after winning 48 Grand Tour stages during his 19-year career, has denied any wrongdoing but has been notified of his suspension by the UCI for the "use of a prohibited method" in 2012 and 2013.

The three other riders to be suspended on Wednesday for the same offence are Slovenian duo Borut Bozic and Kristijan Koren and Croatia's Kristijan Durasek.

Bozic retired last year but now works for Bahrain-Merida, his last team as a rider. Koren has been riding for the same team at this month's Giro d'Italia but has now been withdrawn from the race.

Durasek has been riding in this week's Tour of California for UAE Team Emirates but he, too, has been sent home.

In a statement, the governing body said: "The UCI and the Cycling Anti-Doping Foundation, the independent body mandated by the UCI to define and lead the anti-doping testing strategy and investigations in our sport, have been in close contact with the sport and state authorities involved in the Aderlass investigation, in particular the World Anti-Doping Agency and the Austrian law enforcement authorities.

"The UCI and the CADF will continue to cooperate with and assist all parties involved in these investigations but, in view of the nature of the ongoing investigation, will not make any further comment at this stage."

Operation Aderlass, as the joint Austrian and German investigation is known, started during the Nordic World Ski Championships in February when several premises were raided in the two countries, arrests were made and blood bags seized.

Among those arrested were five cross-country skiers and Dr Mark Schmidt, the man alleged to be at the centre of what the police described as an "internationally active doping network".

Dr Schmidt, who is understood to be cooperating with the authorities, has previously worked with the Gerolsteiner and Milram cycling teams.

Petacchi's name was first linked to Operation Aderlass in the French and Italian media on Tuesday. This followed Sunday's confession by former teammate Danilo Hondo that he had been a customer of Dr Schmidt's in 2012 and 2013.

While Petacchi continues to deny blood doping, Wednesday's news brings the number of riders suspended by the UCI for alleged links to Dr Schmidt to six, as Austrian pair Stefan Denifl and Georg Preidler were suspended in March, and that does not include former German champion Hondo.

The authorities have said there are 21 athletes under investigation in Operation Aderlass, which means 'blood letting' in German.

Reports in France and Italy have suggested that 10 are cyclists, although none of the riders yet to be named are still active.

It has been a difficult week for the sport as Colombian rider Juan Sebastian Molano was withdrawn on Tuesday from the Giro, the first Grand Tour of the season, by UAE Team Emirates after he returned "unusual physiological results" in internal testing.

The 24-year-old is suspended pending further tests by the team and CADF.