An experimental vaccine to prevent the most common forms of cervical cancer proved 100 percent effective in a two-year test on more than 10,000 girls and women, drug maker Merck & Co. says.

Merck is hoping to win Food and Drug Administration approval for the vaccine, Gardasil, and put it on the market as soon as late 2006. It would be the first vaccine to prevent cervical cancer, a disease caused almost exclusively by a highly common sexually transmitted virus called the human papilloma virus, or HPV.

Doctors expect the vaccine to be routinely offered to girls and boys before they become sexually active.

“I see this as a phenomenal breakthrough,” said Dr. Gloria Bachmann, director of the Women’s Health Institute at Robert Wood Johnson Medical School in New Brunswick, N.J.

Worldwide, cervical cancer is one of the most common cancers among women. It kills nearly 300,000 a year, including about 3,700 in the United States. About 20 million Americans have some form of HPV, which in addition to cervical cancer can cause painful genital warts.

The genetically engineered vaccine prevents cervical cancer by blocking infection from the two strains of HPV that cause 70 percent of all cases of the disease.

The study included 10,559 sexually active women ages 16 to 26 in the United States and 12 other countries who were not infected with either of the two virus strains, called HPV 16 and 18. Half got three vaccine doses over six months; half got dummy shots.

Among those still virus-free after the six months, none of those who received the vaccine developed either cervical cancer or precancerous lesions during two years of follow-up, compared with 21 of those who got dummy shots.

The study, which was funded by Merck, will be presented today at a meeting of the Infectious Diseases Society of America.