Massive deficits could force the U.S. Postal Service to cut out one day of mail delivery a week, the postmaster general told Congress on Wednesday as he asked lawmakers to lift the requirement that the agency deliver mail six days a week.

'We could experience a net loss of $6 billion or more this fiscal year.'— U.S. postmaster general John Potter

If the change happens, that doesn't necessarily mean an end to Saturday mail delivery in the United States. Previous post office studies have looked at the possibility of skipping some other day when mail flow is light, such as Tuesday.

The postal service, faced with dwindling mail volume and rising costs, was $2.8 billion in the red last year.

"If current trends continue, we could experience a net loss of $6 billion or more this fiscal year," postmaster general John Potter told a Senate homeland security and governmental affairs subcommittee on Wednesday.

Total mail volume was 202 billion items last year, more than nine billion fewer than the year before, the largest single volume drop in history.

The change to five-day delivery wouldn't necessarily happen right away, Potter said, adding that the agency is working to cut costs and any final decision on changing delivery would have to be made by the postal governing board.

A shift away from first-class letters to the internet for everything from personal letters to bill payments has hurt the post office, Potter said.

Standard mail, largely advertising, had made up much of the revenue difference until the poor economy cut into its use as well, he said.

It currently costs 42 cents US to mail a letter within the U.S.