D.C. Mayor Marion Barry said yesterday it was an "inescapable reality" that a new prison will be built in the District and urged a prison study commission to reconsider its Aug. 26 recommendation not to build the facility.

"That we will have a new facility is a given," the mayor said in a letter delivered to the panel during a meeting. "What is variable is the nature and extent of the commission's role in the planning and design of this facility."

Barry said there is a "strong likelihood" that $10 million will be included for a new prison in the District's 1986 budget and another $20 million the next year.

The prison commission earlier had voted 10 to 3 to recommend expanded alternatives to incarceration, including intensive probation, greater reliance on pretrial diversion and use of D.C. General Hospital to treat convicted drug abusers.

The commission voted yesterday to schedule a meeting next week to consider Barry's request that it reverse its position. The Rev. Edward A. Hailes Sr., chairman of the commission, said he hopes the mayor will attend.

The commission's earlier action was based in part on an assumption that D.C. General Hospital had space to devote to treatment of drug abusers. In addition, several commissioners had stated that the District's ability to reduce the population of the D.C. Jail, in response to a U.S. District judge's order, showed the city could find alternatives to prison under duress.

Barry took issue with both assumptions in his letter. Asserting there is no room for such a program at D.C. General, Barry said the commissioner's recommendation would "actually necessitate construction of a facility on the grounds of D.C. General Hospital."

He added that compliance with Judge William B. Bryant's court order to reduce the jail population fully taxes the District's existing alternatives to prison and therefore reduces the city's "ability to pursue alternatives to incarceration on a wholesale basis."

Barry earlier this year reversed his longtime opposition to building a prison in the District. Stipulating that a new facility be constructed on federal property and with federal funds, the mayor has moved into line with Sen. Arlen Specter (R-Pa.), chairman of the Senate appropriations subcommittee on the District, who has advocated construction.

In July Specter added $30 million for the prison to the District budget over the next two years. The budget bill has passed the Senate appropriations committee and the full Senate is expected to vote on it sometime this month. After that, the bill will go to a House-Senate conference committee.

The Correctional Facility Study Commission, whose 15 members were named to a six-month term on July 16, was formed to examine the question of whether to build a new institution and, if so, where. Composed of seven mayoral appointees and eight members appointed by the City Council, the panel has heard testimony from experts in criminal justice and advocates on both sides of the prison construction issue.

Barry, in urging the commission to reconsider its recommendations, said he envisioned the construction of a prison as only a part of a "multi-faceted assault on crime" that would also include many of the alternatives endorsed Aug. 26 by the commissioners.