Absent unanimous approval among the members of the European Union, an accord would stall. Establishing a minimum tax would require an E.U. directive, and directives require backing by all 28 countries in the union. Ireland had previously hinted that they would object to or block a directive and Hungary could prove to be an even bigger hurdle given its fraught relationship with the union, which has pressed Hungary on unrelated rule-of-law and corruption issues.

Prime Minister Viktor Orban of Hungary has stated that taxes are a sovereign issue and recently called a proposed global minimum corporate tax “absurd.” Hungary’s low corporate rate of 9 percent has helped it lure major European manufacturers, especially German carmakers including Mercedes and Audi.

Bruno Le Maire, France’s finance minister, said on Saturday that it was important that all of Europe supports the proposal. G20 countries plan to meet with Ireland, Hungary and Estonia next week to try and address their concerns, he said.

“We will discuss the point next week with the three countries that still have some doubts,” he said. “I really think the impetus given by the G20 countries is clearly a decisive one and that this breakthrough should gather all European nations together.”

Policymakers also have yet to determine the exact rate that companies will pay, with the United States and France pushing to go above 15 percent, and negotiations are continuing over which firms will be subject to the tax and who will be excluded. The framework currently exempts financial services firms and extractive industries such as oil and gas, a carve-out that tax experts have suggested could open a big loophole as companies try to redefine themselves to meet the requirements for exemptions.

Domestic politics could also pose hurdles for the countries that have agreed to join but need to turn that commitment into law, including in the United States, where Republican lawmakers have signaled their disapproval, saying the plan would hurt American firms. Big business interests are also warily eyeing the pact and suggesting they plan to fight anything that puts American companies at a disadvantage.

“The most important thing is understanding that if there is going to be an agreement, that there cannot be an agreement that is punitive toward U.S. companies,” said Neil Bradley, the chief policy officer at the U.S. Chamber of Commerce. “And that, of course, is of great concern.”