WADA told the International Cycling Union (UCI), which set up the inquiry, that it is unhappy that witnesses are not being offered immunity to encourage them to come forward with information following the sport's biggest doping scandal.

The U.S. Anti-Doping Agency also expressed concerns Tuesday that no truth and reconciliation commission has been implemented by the UCI to ensure riders don't fear retribution from any future confessions.

After years of denials, Armstrong is said to have confessed to doping during an interview with Oprah Winfrey taped Monday.

Armstrong was stripped of his seven Tour de France titles and banned for life from Olympic sports following a USADA report that portrayed him as a longtime user of performance-enhancing drugs.

The UCI has set up what it says is an independent panel to look into claims that it covered up suspicious samples from Armstrong, accepted financial donations from him and helped him avoid detection in doping tests.

In a statement, WADA said it has "shared a number of serious concerns as to the commission's terms of reference and its ability to carry out its role without undue influence."

"WADA is concerned that the scope of the inquiry is too focused on sanctioned former cyclist Lance Armstrong -- especially as his case is closed and completed with there being no appeal -- and will therefore not fully address such a widespread and ingrained problem," the agency stated.

The three-member commission will be chaired by retired British judge Philip Otton and also feature Paralympic great Tanni-Grey Thompson and Australian lawyer Malcolm Holmes.

The panel will meet in London from April 9-26, and deliver its report to the UCI by June 1 -- a deadline which WADA claims is "wholly insufficient and will result in a lost opportunity to properly investigate the problem."

In September 2012 UCI President Pat McQuaid said he was considering an amnesty for riders and officials who confess to doping offenses.

But that hasn't been implemented for the Armstrong investigation.

"Because the commission does not offer immunity there is no incentive for witnesses to come forward, or to even give witness statements," WADA said. "An approach that does not allow individuals to give evidence without the fear of retaliation will merely perpetuate the omertà that has been an obstacle to cycling investigations in the past."

The U.S. Anti-Doping Agency had told the UCI that having a form of truth and reconciliation and amnesty program was crucial for "the future of clean sport."

"UCI's refusal to agree to allow a limited opportunity for riders to come forward and be truthful without fear of retribution or retaliation from the UCI obviously calls into question the UCI's commitment to a full and thorough investigation and creates grave concern that the UCI has blindfolded and handcuffed this independent commission to ensure a pre-determined outcome," USADA chief executive Travis Tygart said.

"The current terms of reference are not good for clean athletes or moving this sport forward to a better future."

See also The media's role in the clean-up and reform of the UCI and cycling (4 pages).