Moving to defuse a politically explosive issue, Maryland Acting Gov. Blair Lee III said today he would scrap controversial plants to build a new prison in Baltimore if prison opponents can find an "acceptable" alternative site and win approval for the new location in this session of the legislature.

Lee's announcement at an informal press conference represents a shift in his previously adamant stand to go ahead with plans to construct a 890 bed state prison on the old Continental Can building site in East Baltimore, despite persistent and heated protest from influential local politicians and residents of the community.

While conceding that he would be "a little surprised" if state prison officials find a worthy alternative site before the session ends in April, Lee gave at least temporary hope to a large segment of the Baltimore legislative delegation, whose cooperation is important in the smooth running of a General Assembly session.

The decision could also pay political dividends in next fall's Democratic primary where Lee is running for governor. Even if he ultimately rejects proposals for a new site and goes ahead with present plans, Lee can at least say he gave opponents of the Continental Can location a chance to find a better site.

"I think if he can (change sites), it removes from him a big political problem," observed Sen. Harry J. McGuirk, a powerful member of Baltimore's delegation, who has thrown his weight behind the prison opponents. "It shows the people (of East Baltimore) he has given consideration to their concerns."

Lee, who is actively seeking the help of Baltimore political organizations for his primary race, denied that he changed his position to enhance his political prospects. "I haven't consulted the politics of the matter much," he said. "There's some things you can get political with . . . This isn't one of them."

A change in prison sites, Lee said, would "probably end up wasting some money." The state has already spent $4.2 million in purchasing the 25-acre site, demolishing the existing buildings and performing architectural and engineering designs for a medium-security prison.

In the waning moments of last year's Assembly session, lawmakers approved the $2.9 million purchase of the Continental Can site from the cousin of then-Gov Marvin Mandel's closest friend, Irvin Kovens. The property owner, Morton Sarubin, made $1 million profit on the land.

The legislature also approved a Mandel administration measure to spend another $23 million to convert the existing structure into a prison. Lee decided last fall to change plans, favoring the construction of a new penitentiary on the same site, instead of renovating the old structure.

Since he decided to build a new prison and ordered emergency measures to speed up the plan, Lee has been under nearly constant pressure from neighborhood organizations and powerful East Baltimore politicians to find another site.

Staunchly defending his unpopular stand as the only reasonable way to relieve overcrowding of state prisons, Lee has said numerous times that there is no feasible alternative to the Continental Can site. Just three months ago, he said at a press conference, "The time for fun and games, for dilly-dallying, for stalling, is over. We've go to move."

At his meeting with reporters in the governor's office today, Lee said he has reconsidered his positions. "If a switch would cost us six or nine months," he said, "it would be really unreasonable to say we can't do it. If we get into a two-year delay. I'm not buying it. We really need this for 1980."

It is up to the Baltimore politicians who are leading the fight against the Continental Can location. Lee said, to find alternative proposals and make them known to a state site-selection panel. The panel has rejected 54 possible prison locations in the last two years.