(CNN) -- Champion cyclist Lance Armstrong won't be allowed to compete in international Ironman triathlon contests while he's under investigation for doping, the World Triathlon Corporation announced Thursday.
The ruling bars him from the Ironman France competition scheduled for June 24 in Nice, the agency said.
The decision comes after the U.S. Anti-Doping Agency, or USADA, confirmed Wednesday that it is opening proceedings against Armstrong and five former teammates.
Armstrong said Wednesday the agency intends to "dredge up discredited" doping allegations against him in a bid to strip him of his seven Tour de France victories.
The champion cyclist has always insisted he never took performance-enhancing drugs.
Armstrong was accused of using such drugs by other riders, but never failed a drug test.
Armstrong started doing triathlons after retiring from cycling, and won a half-Ironman in Hawaii less than two weeks ago. He was the fastest cyclist and runner on the 70.3-mile course, and the third-fastest swimmer.
But with the new investigation open, the World Triathlon Corporation suspended him.
WTC rules "dictate an athlete is ineligible to compete during an open investigation. Armstrong is therefore suspended from competing in WTC-owned and licensed races pending further review," the organization said in a statement.
USA Triathlon said he can still compete in its events.
"Lance Armstrong is currently still eligible to compete in all USA Triathlon-sanctioned races, with the exception of Ironman events as stated earlier by the World Triathlon Corporation," said the body's communications director Chuck Menke.
Armstrong lashed out at his accusers and the doping agency on Wednesday.
"Unlike many of my accusers, I have competed as an endurance athlete for 25 years with no spike in performance, passed more than 500 drug tests and never failed one," Armstrong said on his website. "That USADA ignores this fundamental distinction and charges me instead of the admitted dopers says far more about USADA, its lack of fairness and this vendetta than it does about my guilt or innocence."
A 15-page agency letter was sent Tuesday to Armstrong and several others. The Washington Post was the first to report the story.
Justice Department prosecutors in February said they closed a criminal investigation after reviewing allegations against Armstrong. They had called witnesses to a federal grand jury in Los Angeles, but they apparently determined they lacked evidence to bring a charge that Armstrong used performance-enhancing drugs.
"These are the very same charges and the same witnesses that the Justice Department chose not to pursue after a two-year investigation," Armstrong said Wednesday.
In the letter, a copy of which was obtained by CNN, the U.S. Anti-Doping Agency said it collected blood samples from Armstrong in 2009 and 2010 that were "fully consistent with blood manipulation including EPO use and/or blood transfusions."
EPO, or erythropoietin, boosts the number of blood cells.
The letter said Armstrong and five former cycling team associates engaged in a doping conspiracy from 1998 to 2011. Riders will testify that Armstrong used EPO, blood transfusions, testosterone and masking agents, and that he distributed and administered drugs to other cyclists from 1998 to 2005, according to the letter.
In a statement obtained by CNN, U.S. Anti-Doping Agency CEO Travis T. Tygart said the five other people, formerly associated with the U.S. Postal Service pro cycling team, are three team doctors and two team officials. The letter included written notice of alleged anti-doping rule violations, Tygart said
"USADA only initiates matters supported by the evidence," Tygart said. "We do not choose whether or not we do our job based on outside pressures, intimidation or for any reason other than the evidence." All named individuals are presumed innocent until proven otherwise, he added.
According to its website, the quasi-government agency is recognized as the official anti-doping agency for Olympic, Pan American and Paralympic events in the United States.
Armstrong said the organization has "self-written rules" and punishes first and adjudicates the facts later.
Armstrong has been dogged by allegations of drug abuse in recent years, with compatriot Floyd Landis -- who was found guilty of doping in the 2006 Tour de France, resulting in him being stripped of the title -- making a series of claims last year.
Armstrong came out fighting once again in May 2011 in the face of fresh allegations made on the CBS News "60 Minutes" show by another American, Tyler Hamilton.
In the CBS interview, Hamilton -- who retired in 2009 after twice testing positive himself -- says he first saw Armstrong use blood boosting EPO in 1999, the year of his first Tour de France victory.
"I saw it in his refrigerator," Hamilton told the American news program. "I saw him inject it more than one time like we all did, like I did many, many times."
On his Twitter page, Armstrong at the time said he never failed a drug test.
CNN's Steve Almasy, Gary Morley, Terry Frieden and Jason Durand contributed to this report.