UPDATE 1-GSK tests intravenous flu drug vs Tamiflu

* Phase III study of hospital patients using IV zanamivir

* Zanamivir already sold in inhaled form as Relenza (Adds details, background)

LONDON Jan 19 (Reuters) - GlaxoSmithKline (GSK.L) has started a pivotal study testing intravenous zanamivir against Roche's (ROG.VX) best-selling pill Tamiflu as a treatment for patients hospitalised with influenza, it said on Wednesday.

Intravenous zanamivir is not currently approved for sale in any country but the medicine has been available since 1999 in inhaled form, under the brand name Relenza. GSK licensed the product from Australia's Biota (BTA.AX).

GSK's Phase III trial aims to enrol 462 patients in 20 countries and will take around three years to complete. (Reporting by Ben Hirschler, editing by Kate Kelland)