The Washington Post Co. and the American Radio-Telephone Service Inc. (ARTS) announced yesterday they have combined forces in an effort to win a government license to offer a brand new portable telephone system to residents in the Washington and Baltimore area.

The agreement could give the Post Co. and ARTS, a radio-paging company that is already providing experimental portable phone service in Baltimore and Washington, a significant advantage in winning the government's permission to begin providing this revolutionary "cellular" telephone service.

One industry expert noted yesterday that the agreement combines the engineering and technical strength of ARTS with the financial strength of The Washington Post to make the venture a "formidable application" that could be hard to beat.

The details of the settlement are still private.

The agreement reduces to four the number of companies competing to win the sole nontelephone license for cellular service in this area.

A Washington Post Co. spokesman said "discussions have been held and future discussions are anticipated" with the other applicants for this area to see if they want to join a venture to provide cellular radio service. Winning an agreement among all companies would mean that the competitors could avoid lengthy and costly Federal Communications Commission hearings to determine who should win the government's approval to operate the system.

However, the Post spokesman noted, the company was pursuing its venture with ARTS very seriously with the FCC and planned to win the license in any competitive hearing.

The other companies vying for a license here are: Cellular Mobile Systems of D.C. Inc., a subsidiary of the telecommunications company Graphic Scanning; Metro Phone Associations, a joint venture between Metromedia Inc. and the Alexandria paging and mobile-radio company Advance Radio Communications Co.; Radio Telephone Systems Inc., a new minority-owned firm that specializes in cellular radio.