GlaxoSmithKline's cervical cancer vaccine Cervarix faces a delay in winning approval in the United States, putting it further behind Merck's rival product Gardasil.
Glaxo , Europe's biggest drugmaker, said on Monday it had received a "complete response letter" from the U.S. Food and Drug Administration, which is issued when the regulator has completed the review of a medicine, but still has questions.
Analysts, who had been expecting the FDA to issue a decision on Cervarix by next month, said the setback would delay approval by at least a few months and possibly more than a year.
"GSK will work closely with the FDA to prepare its response, which it intends to submit to the agency as soon as possible," Glaxo said in a statement.
Cervarix is one of Glaxo's biggest new drug hopes and is already approved in 45 countries, including the 27 member states of the European Union.
But it is trailing Merck's Gardasil, which is already well established in the United States -- the world's biggest drugs market.
"This unexpected delay to U.S. approval increases the uncertainty over GSK's top line growth," analysts at Cazenove said in a note.
"Without any real clarity of the potential delay to U.S. approval, this setback will have a significant impact on investor sentiment in our view."
Shares in the group fell 1.4 percent to 13.04 pounds by 0912 GMT.
A Glaxo spokesman declined to comment on when a final U.S. decision on Cervarix might now be made.
In cases such as this, the regulator usually takes up to six months to respond, once it has received the answers to its questions. Glaxo is not disclosing what the outstanding questions are or how long they will take to answer.
Glaxo Chief Executive Jean-Pierre Garnier told the Reuters Health Summit last month he expected Cervarix to eventually secure a 50 percent market share, despite being second to market.
Gardasil is already a blockbuster product, with third-quarter sales of $418 million for Merck. Investors have been betting on eventual multibillion-dollar sales for Cervarix, too, with Garnier describing the size of the potential market as "humungous."
Both Cervarix and Gardasil protect against cancer-causing strains of the sexually transmitted human papillomavirus (HPV) and are designed to be given to girls and young women.
Although similar, the two vaccines are not identical.
Gardasil targets four strains of HPV -- two responsible for cervical cancer and two causing the less serious condition of genital warts.
Glaxo's product addresses only the two cancer strains.