John Kerr, who proposed the amendment, says it's an attempt to "limit the damage" of leaving Britain's biggest market. The Conservative-led government says Britain will leave the EU's customs union and single market after it quits the bloc next year. But some lawmakers and businesses want to retain a customs union to avoid tariffs and border checks.
It's unclear whether the amendment will survive when the bill becomes law. The House of Commons has the power to overturn changes made by the unelected upper house.