The U.S. agency that polices drug use in sports has informed Lance Armstrong that it intends to bring formal doping charges against him, in an action that could ultimately cost the retired cycling champion all seven of his Tour de France titles.

In a 15-page letter reviewed by The Wall Street Journal, the U.S. Anti-Doping Agency, or USADA, told Mr. Armstrong and others that it spoke with "numerous riders, team personnel and others" who "will testify based on personal knowledge" of Mr. Armstrong's alleged doping.

The...