WASHINGTON — The US Postal Service plans to slash its workforce by 20 percent, eliminating 120,000 jobs by 2015 as it struggles to stay solvent, The Washington Post reported late Thursday.

The newspaper, citing internal documents, said the Postal Service's plan is in addition to the 100,000 other jobs it expects to shed through attrition as employees leave the company or retire.

The workforce reduction would come mainly through layoffs, with the remainder through buyouts and early retirements.

But postal union contracts, which prohibits layoffs, would first have to be renegotiated.

"Asking Congress to eliminate the layoff protections in our collective bargaining agreements is an extraordinary request by the Postal Service, and we do not make this request lightly," the management said in a draft document obtained by the Post.

"However, exceptional circumstances require exceptional remedies," it said.

The newspaper also quoted a notice to employees that said: "We will be insolvent next month due to significant declines in mail volume and retiree health benefit prefunding costs imposed by Congress."

The US Postal Service, a public employer that claims to handle nearly 40 percent of global mail, has been piling up losses since early 2008.

Its finances have been severely hit by a decline in volumes amid the rise in Internet use and e-commerce.

The service already has slashed 110,000 jobs, or 16 percent of its workforce, in the past four years.

In July it unveiled a plan to close more than 10 percent of its post offices throughout the United States, and on August 5 it warned that it risked being unable to meet its financial obligations in September.