In a significant shift away from long-prevailing policy in Maryland, Gov. Harry R. Hughes today said the state should not build a new prison, but rather reduce its prison population through expanded community rehabilitation, probation and parole programs.

For the past three years, as previous governors and legislatures debated the issue, the only question was where the new prision should be built, not whether it should be built at all. Maryland has been ordered by a federal court to ease prison overcrowding by July 1980.

Hughes today in a major policy statement, endorsed a task force report commissioned by his newly appointed prisons chief saying that a proposed 890-bed medium security prison should not be built. "This report clearly shows that just building prisons doesn't solve our crime problems," said Hughes. "I not only accept this report, I agree completely with it."

The task force, headed by Judge Robert B. Watts of the Supreme Bench of Baltimore, said Maryland has the fifth highest rate of incarceration in the country -- with 198 inmates for each 100,000 residents -- and has more than twice the rate of the neighboring states of Virginia, Pennsylvania, Delaware and West Virginia.

It was this unusually high rate of imprisonment, according to the report, that was most responsible for the overcrowding in several state facilities that the courts found intolerable and said must be corrected by 1980. There are currently 7,865 inmates and only 5,576 beds.

But the task force said there were several ways to resolve the overcrowding problem without constructing the proposed 890-bed prison. They included:

The construction, now under way, of facilities to accommodate 1,028 prisoners at correctional annexes in Baltimore, Jessup and Hagerstown.

The addition of 125 more probation agents, included in the 1980 budget, which will allow the state to increase its probation rate without overburdening each agent.

Expansion of Community Adult Rehabilitation Centers around the state for minimum-risk prisoners. Currently, although state policy encourages such centers, there are only six in operation and they handle only 255 prisoners. The task force, with the endorsement of Hughes and his newly appointed corrections chief, Gordon C. Kamka, said there should be a concerted effort to reach the original goal of having beds for 2,400 prisoners in these centers.

The increased use by judges of restitution and community service sanctions against offenders, making them repay the victims or severe the community as retributation for their crimes, rather than serve time in prison.

A more liberal use of the governor's powers to commute sentences, particularly for the final 60 or 90 days of a prison term. According to the task force, this could result in a yearly reduction of the prison population by 1,125.

Hughes, who unveiled the report at today's press conference with Judge Watts, Kamka and State Sen. Charles H. Smelser, the legislature's representative on the task force, said he agreed with the direction of the recommendations. He added, however, that the only suggestion he was committed to follow through on was the one to abandon plans for the 890-bed medium security prison.

"What we're proposing here is nothing new," said Watts, a black jurist from Baltimore. "These proposals have been discussed for years, but now we have an administration that is clearly interested in them. The prevailing rhetoric has always been to get tough, to build more jails. But more jails is not always the answer."

Watts said the proposed 890-bed prison would be too expensive -- he estimated the cost at $36 million -- and difficult to manage. He said federal corrections officials recommend that no prison facility have more than 500 beds.

The task force took special note of the high proportion of blacks in Maryland's correctional facilition, but said it was a problem that could not be easily or quickly rectified.