WASHINGTON – Saturday mail may soon go the way of the Pony Express and penny postcards. The Postal Service said Wednesday that it plans to cut back to five-day-a-week deliveries for everything except packages to stem its financial losses in a world radically reordered by the Internet.

“Our financial condition is urgent,” declared Postmaster General Patrick R. Donahoe. But Congress has voted in the past to bar the idea of eliminating Saturday delivery, and his announcement immediately drew protests from some lawmakers. The plan, which is to take effect in August, also brought vigorous objections from farmers, the letter carriers’ union and others.

The Postal Service, which suffered a $15.9 billion loss in the past budget year, said it expected to save $2 billion annually with the Saturday cutback. Mail such as letters and magazines would be affected. Delivery of packages of all sizes would continue six days a week.