Singer Co., the diversified sewing machine and aerospace manufacturer, plans to relocate a substantial portion of a secretive government-contract subsidiary in the new Washington Business Park in Prince George's County. s

Maryland Gov. Harry Hughes has scheduled a news conference in Annapolis this morning with officials of HRB-Singer Inc., to announce that the Singer unit has leased at least 50,000 square feet of space in Lanham for expanded Washington-area operations.

HRB-Singer is engaged in spying technology and holds defense and national security contracts with the federal government. A specialty is tactical and strategic electronic systems for signal collection, analysis, data correlation and airborne reconnaisance.

Area business leaders see the Singer Co. decision to expand here as an indication that many of the nation's miltary contractors will follow suit over the next decade, because of projected major increases in Department of Defense spending under current Reagan administration plans.

Economic development officials in Maryland have been concentrating on such high-technology companies in their recent efforts to attract new companies and jobs to the state. Currently, Maryland ranks 11th among the states in terms of prime defense contract awards, at $1.8 billion a year.

Based in State College, Pa., HRB originally was called Haller, Raymond & Brown Inc. Singer Co. acquired the firm in 1958, in the first of a long series of takeovers that moved the old sewing machine company into a variety of business sectors, including air conditioning and heating, furniture, motor products and government services. Singer, based in Stamford, Conn., was founded in 1863.

According to area business leaders, who asked not to be identified, HRB-Singer will move in late November to a three-story building at the Washington Business Park entrance, now being completed. Initially, HRB-Singer will employ between 100 and 150 professionals at the Prince George's location, but informed sources said that number would grow substantially within a few years.