WASHINGTON, May 5 (Reuters) - President Joe Biden on Wednesday threw his support behind waiving intellectual property rights for COVID-19 vaccines, bowing to mounting pressure from Democratic lawmakers and more than 100 other countries, but angering pharmaceutical companies.

Biden voiced his support for a temporary waiver - a sharp reversal of the previous U.S. position - after a speech at the White House, followed swiftly by an official statement from his chief trade negotiator, Katherine Tai.

“This is a global health crisis, and the extraordinary circumstances of the COVID-19 pandemic call for extraordinary measures," Tai said in a statement, hours after telling an event hosted by the Financial Times: "Time is of the essence."

Shares in a number of makers of vaccines for COVID-19 tumbled on the news. Two of the biggest vaccine makers are U.S. companies - Moderna Inc and Pfizer Inc.

Biden's move won praise from the head of the World Health Organization, Tedros Adhanom Ghebreyesus, who called it a "MONUMENTAL MOMENT IN THE FIGHT AGAINST #COVID19" on Twitter.

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But the pharmaceutical industry's biggest lobby group warned that Biden's unprecedented step would undermine the companies' response to the pandemic and compromise safety.

Biden, who backed a waiver during the 2020 presidential campaign, has made fighting the coronavirus a top priority of his administration and the rollout of vaccines in the United States has led to a decline in case numbers and deaths.

The Democratic president, who campaigned on a promise to re-engage with the world after four years of contentious relations between former President Donald Trump and U.S. allies, has come under intensifying pressure to share U.S. vaccine supply and technology to fight the virus around the globe.

His decision comes amid a devastating outbreak in India, which accounted for 46% of the new COVID-19 cases recorded worldwide last week, and signs that the outbreak is spreading to Nepal, Sri Lanka and other neighbors.

NEGOTIATIONS TO TAKE TIME

Wednesday's statement paved the way for what could be months of negotiations to hammer out a specific waiver plan. WTO decisions require a consensus of all 164 members.

Tai said the United States would participate in text-based negotiations at the WTO to ensure the waiver could happen, but cautioned those deliberations would take time.

The United States and several other countries previously blocked negotiations at the WTO about a waiver proposal led by India and South Africa and aimed at helping developing countries produce COVID-19 vaccines using the IP of pharmaceutical companies.

Tai said: "The Administration believes strongly in intellectual property protections, but in service of ending this pandemic, supports the waiver of those protections for COVID-19 vaccines."

A person familiar with the U.S. consultations on the issue said Wednesday's decision allows Washington to be responsive to the demands of the left and developing countries, while using WTO negotiations to narrow the scope of the waiver. Since the negotiations will take time, the decision also buys time to boost vaccine supplies through more conventional means.

Dr. Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security, said the patent waiver “amounts to the expropriation of the property of the pharmaceutical companies whose innovation and financial investments made the development of COVID-19 vaccines possible in the first place.”

But proponents of the waiver say the pharmaceutical companies will suffer only minor losses because the waiver would be temporary - and they will still be able to sell follow-on shots that could be required for years to come.

(Reporting by Andrea Shalal, Jeff Mason, David Lawder, Steve Holland, Michael Erman and Patricia Zengerle; Editing by Peter Cooney)