The U.S. Postal Service announced plans on Wednesday to drop mail delivery on Saturdays starting in August in an effort to save about $2 billion a year.

The agency, which lost nearly $16 billion last year, will go to a five-day delivery schedule starting the week of Aug. 5. It will continue delivering packages six days a week, as well as mail addressed to post office boxes.

"The Postal Service is advancing an important new approach to delivery that reflects the strong growth of our package business and responds to the financial realities resulting from America's changing mailing habits," Postmaster General Patrick Donahoe said in a statement.

 "We developed this approach by working with our customers to understand their delivery needs and by identifying creative ways to generate significant cost savings."

Labor unions were upset over the announcement. "I think the Postal Service is willfully jumping into a death spiral," said John Marcotte, president of the Michigan Postal Workers Union that represents 6,000 workers in Michigan.

"They continue to lower service, slow down the mail and provide less options for our customers, and I think the loss in revenues will far outweigh any short-term gains in saved expenses."

The Postal Service is already facing some pushback for moving forward with delivery schedule changes without permission from Congress.

"Today's announcement by Postmaster General Donahoe to eliminate six-day delivery is yet another death knell for the quality service provided by the U.S. Postal Service," said Jeanette Dwyer, president of the National Rural Letter Carriers' Association. "To erode this service will undermine the Postal Service's core mission and is completely unacceptable."