Justice Department civil rights attorneys told a federal judge yesterday that the Chicago school board's plan to desegregate the nation's third-largest school system is inadequate and should be redrawn to comply with an existing settlement order.

In an 82-page response filed in Chicago, the department lawyers cited several areas where the school board plan failed to meet the requirements of the consent decree its members signed last fall to settle the long-standing controversy. Noting that the decree's goal was to "establish the greatest practicable number of stably desegregated schools," the department said the board had made "no attempt to meet this burden."

Members of the civil rights community have been watching for the Reagan Justice Department's response to the Chicago school board plan as a sign of the new administration's approach to civil rights enforcement. Michael H. Sussman, who worked on the case of Justice and was critical of Attorney General William French Smith's rejection of busing as a desegregation remedy, said yesterday that he was encouaged by the department's rejection of the Chicago plan.

In its filing, the department lawyers listed the following examples of the plan's inadequacies. It doesn't:

Take effect this fall, as the settlement requires.

Provide justification for why some schools remain substantially segregated.

Offer any rationale for why it would allow a maximum 70-30 ratio of whites to minorities in a school system where only 18 per cent of the students are white. "The Chicago Board has selected an arbitrary 'mathematical' ratio and applied it without regard to the practicalities."

Consider the relatively severe isolation of the students in 231 elementary schools that are virtually all-black.

Explore a full range of voluntary techniques, but relies on those that haven't been successful in the past in desegregating schools.

Provide adequate mandatory backup in case the voluntary efforts fail.

Because of the failings in the plan, the department urged the court to order the school board to try again