Royal Dutch Shell Plc

Sanctions on Iran could cost US, European companies billions

European and American companies could lose billions of dollars in commercial deals canceled and a major new export market undercut by the U.S. decision to re-impose sanctions on Iran.

President Donald Trump's decision will likely most hurt aircraft makers, oil companies and auto manufacturers. The final impact, however, remains unclear because of possible exemptions for some companies and even new negotiations on a revised Iran nuclear agreement, experts say.

Since the Iran nuclear accord was struck in 2015, it has been mostly European companies that moved back in to sign deals with Iran. Europe traditionally has had closer business and diplomatic ties than the U.S., where...