The sale of individual mixed drinks to go, long a staple of the retail liquor businesses in St. Mary's County, has been declared illegal starting today by the county liquor board.

"Go cups," as they are called in Southern Maryland, were cited in a critical grand jury report in October as a contributing factor in the high number of drunk driving fatalities in St. Mary's County.

State police say the rate in the county exceeds the state average by 25 percent.

This year 11 of the 13 auto fatalities in the county were alcohol-related, police reported.

Last year, alcohol was listed as the major factor in 12 of the 17 auto deaths there, according to Lt. James Plunkett of the state police.

Statewide, alcohol was a factor in 51 percent of 1983's fatal accidents, he said.

The three-member Alcoholic Beverages Board passed the resolution this week because of community concern about "the proliferation of places offering these open containers of mixed drinks," said Michael V. Davis, attorney for the liquor board.

Some liquor retailers told the board they felt compelled to offer cups and ice with every bottle they sold because their competitors did.

Davis said he is not sure the new regulation will cover this practice but said county bartenders who continue the long-standing practice of selling customers one more drink to go at closing time will be subject to fines and suspension of their liquor licenses.

Davis said that while it will be up to the liquor board to determine how broadly the new rule is interpreted, "It is my feeling that any open drink sold in a little plastic cup and intended to be taken off the premises will be covered."

"This is not just a holiday moratorium," Plunkett said. "This is it for 'go-cups.' "