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By Patricia Zengerle and Richard Cowan

WASHINGTON, May 19 (Reuters) - U.S. Senate Democratic leader Chuck Schumer on Monday introduced a bill that would prevent any foreign aircraft from being used as Air Force One, the aircraft that transports the U.S. president.

Schumer introduced the "Presidential Airlift Security Act" after news last week that President Donald Trump planned to accept a $400 million airplane from Qatar, which legal experts said raised a raft of questions about the scope of laws that relate to gifts from foreign governments and are intended to thwart corruption and improper influence.

Retrofitting the luxury plane offered by Qatar's royal family would require security upgrades, communications improvements to prevent spies from listening in and equipment to fend off incoming missiles, experts said.

The costs were not known, but could be significant given that Boeing Co'sBA.N current effort to build two new Air Force One planes exceeds $5 billion.

The bill introduced by the New York Democrat would prohibit the Department of Defense from using any funds to procure or modify foreign aircraft in order to serve as the president's air transportation.

The measure is unlikely to become law, given that Trump's Republicans hold majorities in the Senate and House of Representatives, which would both have to approve it and override a veto if Trump objected.

But it marked another effort by Democrats to oppose the plan. Schumer last week vowed to block all of Trump's nominees to the Justice Department until the agency reports what it knows about Qatar's offer.

Multiple congressional Republicans also raised concerns about Qatar's offer.

Trump said it would be "stupid" for him to refuse the offer.

(Reporting by Richard Cowan and Patricia Zengerle, Editing by Franklin Paul)

((patricia.zengerle@thomsonreuters.com, www.twitter.com/ReutersZengerle; 001-202-898-8390;))