Gov. Harry Hughes is expected to call soon for construction of a new, medium-security state prison, formally putting aside the reformist corrections policies of the first half of his administration.

The governor's support for a major new prison became "almost certain" today, according to sources, as a Hughes-appointed task force on prisons concluded in a formal report that the crowded and troubled corrections system needs a new facility with at least 500 beds to house its growing population through the 1980s.

Hughes has told members of his staff that he would accept the task force's recommendation, the sources said. He is expected to include the new prison, which probably would cost between $35 million and $50 million, in his 1982-83 budget request to the legislature in January. It still is unclear where the prison would be located, although corrections officials and sources close to Hughes consider Hagerstown the most likely site.

Del. Frank Robey Jr. (D-Baltimore), who chairs the House budget panel that oversees prisons, said today he may push to locate the prison in Prince George's County, a move that would set off a fierce political struggle in the 1982 session.

Today's vote by the task force of legislators and executive staffers capped five months of gradual movement in Maryland toward more strict prisons policies, following the resignations under fire of former Public Safety Secretary Gordon Kamka and his corrections commissioner, Edwin Goodlander.

Kamka and Goodlander had argued, with Hughes' support, that Maryland could solve its severe prison overcrowding problems by putting more inmates in community-based centers and by accelerating paroles. Those policies, unpopular from the start with legislators, came under intense fire last spring after 27 inmates from a work-release program were arrested on various charges including murder, rape, robbery, heroin-trafficking and other crimes while they were supposed to be attending jobs or college classes. Kamka and Goodlander resigned days after the arrests.

Under Kamka's successor, Thomas W. Schmidt, the lenient policies have been disbanded. New guidelines disqualify all but a small pool of prisoners for community programs like work release, and more prisoners are being assigned to the already-crowded medium- and maximum-security prisons.

Hughes opened the door for construction of a new prison less than a week after the highly publicized arrests of the 27 inmates. He struck a compromise with legislators to appropriate $800,000 in planning money for a new, 500-bed prison. The money would not be spent, Hughes stressed, unless a task force appointed by him concluded after considerable study that the new facility was needed.

That conclusion came today. The task force report, approved in final form, said Maryland's prison population will peak at 9,800 in this decade, meaning the state needs 1,020 more beds than the 8,600 already built or in various stages of planning and construction.