The strategy would effectively be a constitutional challenge to the debt limit. Under the theory, the government would be required by the 14th Amendment to continue issuing new debt to pay bondholders, Social Security recipients, government employees and others, even if Congress fails to lift the limit before the so-called X-date.

Ms. Yellen, however, continued to dismiss that notion.

“There would clearly be litigation around that; it’s not a short-run solution,” Ms. Yellen said at a news conference in Japan before a meeting of finance ministers from the Group of 7 nations. “It’s legally questionable whether or not that’s a viable strategy.”

Biden administration officials have studied the idea, but the president also voiced similar skepticism this week after meeting with Speaker Kevin McCarthy and predicted that unilateral action to raise the debt limit without Congress would spur litigation.

As she prepared to meet with her international counterparts, Ms. Yellen warned that failing to lift the debt limit would have dire consequences for the United States and the world economy. She noted the significant uncertainty associated with a default but predicted that a sharp decline in government spending combined with the expected turmoil in financial markets would lead to a “very substantial downturn.”

“A default would threaten the gains that we’ve worked so hard to make over the past few years in our pandemic recovery,” Ms. Yellen said. “And it would spark a global downturn that would set us back much further.”