The Federal Home Loan Bank Board yesterday announced that its policies permit interstate branching in cases of failing federally insured savings and loan associations that are involved in supervisory or involuntary mergers.

Industry sources say the first such interstate supervisory merger is likely to occur in the Washington area. A number of the largest District S&Ls are expected to make bids soon for one of several suburban savings and loans that reportedly are in weak financial condition.

Thus, the District S&Ls, which took the lead originally in petitioning federal regulators to relax their ban against interstate branching and then saw their hopes dashed, finally would be able to branch into the suburbs, but only over the moribund frames of other S&Ls.

On the surface, the quick, late-afternoon vote appeared to leave unaltered the board's attitude on the controversial issue of acquisitions across state lines. Chairman John H. Dalton declared, "Today's action does not signal any change in attitude or direction on the part of the bank board with respect to the issue of interstate branching."

Privately, informed sources said the move on the part of the outgoing chairman was a reaction to Sen. Jake Garn's opposition to interstate branching. Soon after Dalton became chairman last November, he announced a renewed interest in interstate branching.

Garn, who is chairman of the Senate Banking Committee, does not favor interstate branching. He exacted a promise from Dalton that he would not proceed on the question until the Reagan administration had evolved its policy and Garn had held hearings.

Dalton is to be replaced soon by Richard Pratt, who has Garn's support. So the restatement of the bank board's policy with regard to supervisory mergers, with accompanying guidelines, was seen by some observers as Dalton's revenge.

"I strongly disapprove of the bank board's change in branching policy," Garn said yesterday, adding that he believes Congress, rather than a federal regulatory agency, should decide such changes.

The guidelines offer broad exceptions to the standing policy against interstate mergers. They allow such actions when the board determines interstate mergers would reduce the insurance liability of the Federal Savings and Loan Insurance Corp., which insures S&Ls, more than intrastate mergers would.

The number of involuntary mergers of troubled thrifts arranged by the FSLIC is expected to increase dramatically this year as the industry experiences its worst year ever. There is concern that the assets of the FSLIC, totaling $6.5 billion, may be sorely strained by arranging a large number of mergers.