(Adds Sanofi-Aventis contribution, comment)

LONDON, June 13 (Reuters) - GlaxoSmithKline Plc <GSK.L> will donate 50 million doses of its "pre-pandemic" bird flu vaccine for humans to a global stockpile for distribution in the world's poorest countries.

Europe's biggest drugmaker said on Wednesday it would deliver the vaccine -- enough for 25 million people -- to the World Health Organization (WHO) over a three-year period.

The company's chief executive, Jean-Pierre Garnier, also urged the governments of major European countries such as Germany, France, Italy, Spain and Britain to place orders to secure a supply for their populations.

Glaxo's vaccine, which is made with a proprietary adjuvant, or additive, targets the H5N1 virus that has killed around 190 people among more than 300 known cases since 2003.

French drugmaker Sanofi-Aventis <SASY.PA> said on Wednesday that it too is ready to supply a significant number of doses of H5N1 vaccine for an international stockpile through a WHO partnership.

Part of this supply could be made available immediately, in bulk form, Sanofi said.

While H5N1 remains mainly a virus affecting birds, scientists say it is the most likely source of the next global flu pandemic in humans, since it may soon mutate into a form transmitted easily from person to person.

Garnier said Glaxo's pre-pandemic shot would give "a degree of protection" until a precisely tailored pandemic vaccine could be produced -- a process likely to take four to six months from the time a pandemic strain is identified.

"This is another significant step toward creating a global resource to help the world and especially to help developing countries in case of a major outbreak of H5N1 avian influenza," WHO director-general Margaret Chan said.

Sanofi, Swiss drugmaker Novartis AG <NOVN.VX> and Baxter International Inc. <BAX.N> are all working on bird flu vaccines, which industry analysts say represent a $1 billion- plus sales opportunity for companies.

The United States and some smaller countries have already placed orders for national stockpiles, but there has been concern the world's poor could be left without protection.

As a result, the WHO has decided to set up a vaccine stockpile to distribute shots at short notice to poor countries.

Sanofi Chairman Jean-Francois Dehecq said his company was taking further steps to assist in that effort.

"Above and beyond this vital initiative with the international community and with WHO, we are actively pursuing further moves that are intended to increase influenza vaccine supply for countries in the developing world, enabling us to provide a meaningful contribution to global pandemic preparedness," Dehecq said in a statement.

Garnier told reporters Glaxo was also ready to sell stocks at preferential prices to the nonprofit GAVI Alliance, which could result in tight global supply.

"GAVI might very well buy significant units for the developing world and, if they come first, we will serve them first," he told reporters in a conference call.

"None of the big (European) countries -- whether it's Germany, France, Italy, Spain, UK -- have come forward and made a final determination ... we hope they are going to come to a final decision because at some point we have to apportion our production," he added.

Glaxo's pre-pandemic vaccine has yet to be formally cleared by regulators in the United States and Europe, but Garnier said authorities were likely to speed its approval in the event of a global flu crisis.

The product has a shelf-life of three to five years. (Additional reporting by Bill Berkrot in New York)

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