The European Union and Japan have signaled that they plan to announce a broad agreement on trade on Thursday, a pointed challenge to President Trump, who is scheduled to attend a meeting of world leaders in Germany the next day.

The timing of the announcement — on the eve of the Group of 20 summit meeting in Hamburg, Germany — was a clear reaction to the United States’ protectionist stance the last time the G-20 met. During a meeting in March of cabinet-level officials in Baden-Baden, Germany, Steven Mnuchin, the American Treasury secretary, refused to endorse a statement in favor of free trade.

By forging ahead with their own accord before the meeting with Mr. Trump and other heads of state, Europe and Japan threatened to isolate the United States in important industries like automobiles.

“Ambitious free and fair trade deal in the making,” Donald Tusk, the president of the European Union, tweeted on Monday.