LONDON—A group of the world’s richest nations reached a landmark deal on Saturday to close cross-border tax loopholes used by some of the world’s biggest companies. The Group of Seven said it would back a minimum global corporation tax rate of at least 15 percent, and put in place measures to ensure taxes were paid in the countries where businesses operate. “After years of discussion, G7 finance ministers have reached a historic agreement to reform the global tax system to make it fit for the global digital age,” British finance minister Rishi Sunak told reporters. The accord, which could form the basis of a global pact next month, is aimed at ending a decades-long “race to the bottom” in which countries have competed to attract corporate giants with ultra-low tax rates and exemptions. That has in turn cost their public coffers hundreds of billions of dollars—a shortfall they now need …