Glaxo has licensed the treatment from Biota, an Australian biotechnology company, which is to receive a 7 per cent cut of Relenza's sales.
The drug is already on the market in Australia and New Zealand, and has been approved in all of the countries of the European Union. It will be launched in the US and UK this autumn. Glaxo's costs in taking the treatment through clinical trials are thought to have totalled pounds 400m.
Relenza works by attempting to arrest the flu virus, which resides in the throat and lungs, at source. It prevents it spreading from one cell to another within the respiratory tract. In clinical trials, flu patients on Relenza suffered less severe symptoms and recovered faster than those on a placebo drug. Glaxo shares yesterday closed up 73p at 1679p.