The eurozone’s economy slowed further in the three months through June, as exports sputtered and business confidence weakened on worries over future relations with the currency area’s largest trading partners.

The loss of momentum in 2018 contrasts with the U.S., where growth accelerated during the second quarter. The U.S. growth differential was the widest since the second half of 2014, the two economies having expanded at roughly the same pace since then.

If...