The Alexandria City Council failed last night to enact a ban on instant bingo, a controversial form of gambling that law authorities have said was a key element of the city's recent bingo scandal and corruption investigations.

"This measure punishes the innocent," said retiring Vice Mayor Nora O. Lamborne (D). "It's is extremely unfair," she said.

Instant bingo is played by tearing a tab off a preprinted card, exposing a winning or losing symbol.

Council member Donald C. Casey (D), who favored the ban, last night called instant bingo "nothing much different than slot machines . . . or playing a dice game. But the 4-to-3 vote went against him.

All of the council members who voted against Casey's proposal will leave office this summer. Casey said he will reintroduce the measure when a new council begins work in the fall.

Bingo is a $1.2 million industry in Alexandria. Games are sponsored by churches and charitable organizations. Several groups recently gave the council petitions opposing the ban on instant bingo.

Last year, city prosecutor William L. Cowhig was acquitted in two trials on illegal gambling and bribery charges in connection with bingo operations. A third charge against him was dropped when he resigned.

Voting against the ban on instant bingo were Mayor Frank E. Mann (I), Lamborne and council members Nicholas A. Colasanto (D) and Ellen Pickering (I).

Voting for it were Casey and council members Beverly Beidler (D) and Robert L. Calhoun (R).