WASHINGTON, July 23 (Reuters) - Under a cost-saving plan by the U.S. Postal Service, millions of Americans accustomed to getting their mail delivered to their doors will have to trek to the curb and residents of new homes will use neighborhood mailbox clusters, the agency said Tuesday.

The Postal Service has been quietly phasing in the change with some aspects starting in April, and it has given no timeline for the shift. It's unclear if delivery to the door will be eliminated entirely.

“Converting delivery away from door delivery to either curb line or centralized delivery would enable the Postal Service to provide service to more customers in less time,” Postal Service spokeswoman Sue Brennan said Tuesday.

More than 30 million American homes get door-to-door delivery and another 50 million get their mail dropped at their curbside mailboxes.

But the Post Service, which is buckling under massive financial losses, sees savings in centralized mail delivery. Door-to-door delivery costs the Postal Service about $353 per address each year.

Curbside delivery costs $224, and cluster boxes cost $160 per address. With cluster boxes, mailboxes for individual addresses are grouped together at a central neighborhood location.

The move is one of many controversial cost-cutting steps the Postal Service is trying as it continues to plead with Congress for permission to overhaul its business and avert a bailout.

The agency also proposed eliminating Saturday mail delivery but was forced to back off that plan earlier this year after lawmakers and some industries balked at the proposal.

Representative Darrell Issa, a California Republican who has spearheaded Postal Service reforms, has introduced a bill that would also require cluster boxes for existing residences with an exemption for people with disabilities.

The legislation will be considered on Wednesday by the House Committee on Oversight & Government Reform.

A senior committee staffer said the legislation could realize annual savings of $4 billion from switching to curbside and $6 billion from cluster boxes.

The Postal Service last year lost $16 billion, mostly due to dwindling mail volumes and massive payments into a mandatory fund for its future retirees' healthcare.

The agency, which does not receive taxpayer funds, is under pressure to modify its business model and raise revenues or risk requiring a bailout of nearly $50 billion by 2017.

PHASING IN

The Postal Service's efforts to phase out direct-to-door mail delivery are being received with trepidation.

Cranberry Township, 25 miles north of Pittsburgh, Pennsylvania, earlier this month received a letter from the local postmaster saying that all new homes would be serviced by cluster box units rather than door-to-door or curbside delivery.

Most new residential construction plans have already been approved and the change would require going back to the drawing board, said Ron Henshaw, the town's director for community development.

“Our board supervisors who actually run the township are very concerned about this,” he said.

Since April, residential developers can no longer choose between curbside or cluster box delivery, agency spokeswoman Brennan said. The Postal Service now makes that determination based on what it considers to be more effective.

The Postal Service has also been asking shopping malls and business parks to voluntarily convert to centralized delivery.

Some in the mailing community such as the Greeting Card Association support a switch to a cluster box system.

But others such as the National Association of Letter Carriers and the American Postal Workers Union oppose it.

Sally Davidow, spokeswoman for APWU, said that moving to cluster delivery would put the Postal Service at a competitive disadvantage against its main competitors, FedEx and UPS , which continue to deliver at the door.