A leading anti-doping expert says he's leaving cycling's biological passport project because of attempts to stop him from speaking publicly about drugs in the sport.

Australian scientist Michael Ashenden told the BBC that an "omerta" was being imposed by the (WADA) World Anti-Doping Agency-accredited laboratory in Switzerland, which manages cycling's anti-doping efforts.

"There should be nothing to hide, so why stop the experts from talking?" Ashenden asked.

Ashenden joined an expert panel analyzing cyclists' blood values for potential doping signs when the International Cycling Union (UCI) launched the biological passport in 2008.

Officials in Lausanne, Switzerland, took over running the project this year and insisted on a confidentiality clause in experts' contracts.