The Justice Department is considering a plan to file a lawsuit to force the busing of students to end the segregation of public schools in Charleston, S.C.

Sources said the plan to sue the Charleston schools is on the desk of Associate Attorney General John H. Shenefield, who must then pass it on to Attorney General Benjamin R. Civiletti for final approval. Sources said the plan has already been approved by the head of the civil rights division, Drew S. Days III, who refused to confirm or deny that the plan exists.

"We have several cases that have been under investigation for some months and may well reach the point before the end of this administration where there's no alternative but to file them," Days said. "We've never taken the position in this administration that when a case was ready to go that we were going to hold it up."

Sources said an investigation of the public school system in Charleston has been under way since March 1979, when civil rights leaders pointed out to the Justice Department that there were as few as five white students in all but one of the public elementary schools in Charleston. The black student enrollment in the same schools is almost 8,000.

"What we have in metropolitan Charleston is a segregated school system," one source said. "The white students are all in the suburban school districts, the blacks are all in the city schools."

Sources said the Justice suit would follow the lines of previous suits that forced busing of public school students in metropolitan Cleveland, Detroit, and Chicago. In those suits, hundreds of thousands of public school pupils were involved in the decision to sue.

Any decision to sue Charleston is sure to involve Sen. Strom Thurmond (R-S.C.), the ranking minority member and next chairman of the Senate Judiciary Committee. Thurmond has never been a champion of busing as a way to achieve racial equality in the public schools anywhere, no less his home state.

A lawsuit to force busing in the Charleston schools could be held up or even dropped by the incoming Reagan administration, especially under pressure from Thurmond. Sources said one reason a decision has not been made in Justice to sue Charleston is the uncertainty over what would happen once the Reagan administration takes over.

The racial makeup of the public schools in metropolitan Charleston has been under fire since 1970, when white students began to move out of the city in large numbers. Critics have said that one reason white students left the city schools was a decision by the old Health, Education, and Welfare Department that allowed each of Charleston's 20 school districts to screen and hire all its teachers.

"The suburban schools ended up with white principals and a large percentage of white teachers," one critic said. "The city schools ended up with mostly black teaching staffs."

Sources said there is only one public school in Charleston with anything close to a racial mix. This school has many as 30 white students in a school population of more than 200.

Maurice Cohen, a white resident of Charleston who moved from New York City in 1975, wrote earlier this year of the experience: "We enrolled our children in public schools. Sarah was the only white child in her kindergarten class; Lottie the only white child in her third grade class; William, entering the seventh grade, was one of three white children in a combined sixth and seventh grade class."

Sources said the Justice Department plan involves a suit that will seek to assign students to schools throughout Charleston County, taking away the authority of single school districts to assign students and teachers.