UPDATE 1-Glaxo wins Japanese approval for Cervarix

* First cervical cancer to win approval in Japan

* Merck's Gardasil awaiting green light

(Adds details, background)

LONDON, Oct 16 (Reuters) - Japanese authorities have approved GlaxoSmithKline's (GSK.L) Cervarix as the country's first cervical cancer vaccine, the British-based drugmaker said on Friday.

The news is a boost for the Glaxo product, which is lagging behind Merck & Co's (MRK.N) rival vaccine Gardasil in Europe and has yet to be approved in the United States.

Gardasil is still awaiting approval in Japan, the world's second largest pharmaceutical market after the United States.

Cervarix's slow path to market in the United States has undermined analysts' initial confidence in its global sales potential and many now doubt it will ever catch up with Gardasil, which was approved by U.S. authorities in 2006.

Consensus forecasts suggest Cervarix will generate worldwide sales of just over $1 billion in 2013, according to Thomson Pharma, equivalent to some 2 percent of expected Glaxo sales for that year.

Global sales of Gardasil were $1.4 billion in 2008, with an additional $865 million received through the Sanofi-Pasteur MSD joint venture partnership with Sanofi-Aventis (SASY.PA).

Both Gardasil and Cervarix protect against infection with the sexually transmitted human papillomavirus (HPV), which causes cervical cancer. (Reporting by Ben Hirschler; Editing by David Cowell)