A senior United States Postal Service official has given the first estimate of how the closing of 82 mail-processing plants will slow letters next year.

In a Sept. 10 video message to the National Postal Customer Council in Salt Lake City, Utah, Megan Brennan, the agency’s chief operating officer, said she expects to “preserve approximately 66 percent of current overnight delivery volumes” after the consolidations are complete next fall.

Her estimate suggests that one-third of the letter mail currently delivered overnight will be delayed at least a day by the planned changes.

Overnight service, she said, will remain available to commercial mail that is “properly prepared, containerized and entered by critical entry times.”

The shuttering of the 82 plants, which has been opposed by 51 senators, should begin in January and conclude by the fall, Postal Service officials said.

The USPS has no choice but to resume the shuttering of processing plants, officials say, because of the sharp decline in first-class mail volume and the costs of maintaining unneeded facilities.

In her Utah speech, Brennan stressed the benefits the USPS would reap from the changes.

“We’ll also be able to provide you with higher levels of reliable and predictable service,” she told the mailers.

“The first thing to know is that these changes will drive sizeable improvements in operating and transportation efficiencies — which minimize the pressure to raise prices,” she said.