WASHINGTON — The Trump administration has formally notified the United Nations that the United States will withdraw from the World Health Organization, a move that would cut off one of the largest sources of funding from the premier global health organization in the middle of a pandemic.

“The United States’ notice of withdrawal, effective July 6, 2021, has been submitted to the U.N. secretary general, who is the depository for the W.H.O.,” a senior administration official said on Tuesday.

The departure would take effect sometime next year, should the United States meet established conditions of giving a one-year notice and fulfilling its current financial obligations, Stéphane Dujarric, a spokesman for the secretary general, António Guterres, said on Tuesday.

The notification completes a threat that President Trump began making months ago, as the death toll from the coronavirus in the United States mounted and Mr. Trump sought to blame the Chinese government for not doing enough to stop the spread of the disease. Mr. Trump has accused Beijing of hiding the true scope of infections from the W.H.O., targeting the agency in the process.

“The world is now suffering as a result of the malfeasance of the Chinese government,” Mr. Trump said in May, when he first said the United States would withdraw from the organization.

There is little evidence to support Mr. Trump’s belief that the Chinese misinformed the organization, though scientists and health experts have recently criticized the W.H.O. for being slow to update its guidance and keep step with science as understanding of the virus rapidly evolves.

Mr. Trump’s decision to leave the W.H.O. is the latest in a series of withdrawals he has made from global pacts, usually after scorning such partnerships as disadvantageous for Americans. But the administration’s move to withdraw from the W.H.O., an organization the United States had a central role in creating more than a half century ago, during a pandemic that has infected more than 11.6 million people, killed more than half a million and upended life around the world, drew swift condemnation from public health experts.

Lawrence O. Gostin, the director of the O’Neill Institute for National and Global Health Law at Georgetown University, said that the decision was “disastrous” for national interests and that the departure would weaken American influence on international health diplomacy.

“President Trump’s official notice of withdrawal from the W.H.O. is among the most ruinous presidential decisions in recent history,” Mr. Gostin, who is also affiliated with the World Health Organization, said in a statement. “It will make Americans less safe during an unprecedented global health crisis.”

Experts acknowledged that the W.H.O. has made some missteps during the pandemic, but said it has largely done well given the constraints under which it operates. The agency is coordinating clinical trials of treatments, as well as efforts to manufacture and equitably distribute the vaccine.

“I think it’s an extraordinarily bad decision that will both harm global public health and harm the health of the American people,” said Dr. Ashish Jha, the director of the Harvard Global Health Institute. “It’s unclear to me how the American people benefit by not being at the table and not being able to shape those policies.”

For the Trump administration to blame the W.H.O. for not investigating the outbreak in China is “deeply disingenuous,” Dr. Jha added. “W.H.O. can’t push its way into China, any more than it can investigate why our outbreak is so bad in Arizona or why we’re botching the response as badly as we are.”

And Dr. Thomas R. Frieden, a former director of the Centers for Disease Control and Prevention, underlined the organization’s accomplishments. “Without W.H.O., the world would not have eradicated smallpox, multidrug resistant tuberculosis would have spread much more widely, and we would have much weaker systems to track influenza and other deadly infections.”

The administration’s move also drew criticism from Democratic lawmakers, including from Senator Bob Menendez of New Jersey, the top Democrat on the Senate Foreign Relations Committee, who wrote on Twitter that Congress had just received notification of the withdrawal. “This won’t protect American lives or interests — it leaves Americans sick & America alone,” Mr. Menendez wrote.

But Republicans on Tuesday supported the idea of leaving the organization.

“Withdrawing the United States from the World Health Organization was the right decision,” Representative James R. Comer of Kentucky, a ranking member of the House oversight committee, said on Twitter. “Until the WHO undergoes some serious reforms, it doesn’t deserve our money or our membership.”

The United States played a central role in creating the W.H.O. in 1948, and has since been one of its largest sources of financial support. The biennial budget for the W.H.O. is about $ 6 billion, which comes from member countries around the world. In 2019, the last year for which figures were available, the United States contributed about $ 553 million.

In the past, the W.H.O. and global health priorities have enjoyed bipartisan support. But in May the Trump administration delivered a four-page letter calling for “major, substantive improvements” in exchange for continued funding.

The organization’s other member nations decided instead to conduct an “impartial, independent” examination of the W.H.O.’s response to the coronavirus pandemic. And it did not satisfy its American critics.

Katie Rogers reported from Washington, and Apoorva Mandavilli from New York.