The Postal Service frequently cannot give customers an accurate estimate of when their mail will be delivered, a government report said.

The report, by the Government Accountability Office, said the service’s delivery standards are out of date.

“According to the deputy postmaster general, some Priority Mail delivery standards call for on-time delivery of Priority Mail in two days, but it is often physically impossible for U.S.P.S. to meet these standards when that requires moving the mail across the country,” the report said.

Priority Mail is advertised by the Postal Service as one of its fastest options.

Postmaster General John Potter said updating delivery standards would increase costs that “would have to be offset by appropriate price adjustments.”