LONDON - GlaxoSmithKline P.L.C.'s Cervarix cervical-cancer vaccine will be delayed in the United States because federal regulators demanded more information, the company said yesterday.

The Food and Drug Administration is evaluating the drug before granting marketing approval.

The vaccine protects girls and women against two strains of the sexually transmitted human papillomavirus that causes cervical cancer. The delay could be six months to two years if the drugmaker needs to submit results from a clinical trial, Panmure Gordon & Co. P.L.C. analyst Savvas Neophytou said in a note to clients.

Cervarix "is seen as the forerunner of all the other big products" in GlaxoSmithKline's pipeline, Neophytou said in an interview. "This is a very important drug for their growth."

The British company, which has a U.S. headquarters in Philadelphia, had said earlier that it expected to start selling the drug in the United States in 2008. Cervarix is expected to become a multibillion-dollar product.

The company received a so-called complete response letter, which the FDA issues when the review of a marketing application file is completed but questions remain to be answered before approval is granted.

"We have already started addressing the questions and will be engaged in discussions with the FDA to finalize our responses," said Barbara Howe, GSK's vice president and director of North American vaccine development.

GSK's American depositary receipts, traded on the New York Stock Exchange, fell $1.40, or 2.64 percent, to $51.60 yesterday.