The U.S. Justice Department today ruled that the majority of Houston's annexations during the last two years violate federal civil rights law by diminishing the strength of minority voting.

The department ordered the city not to hold any elections in which those in largely white-populated annexed areas could vote until its system of electing all city officials at-large is changed.

The decision could force postponement of regular city elections Nov. 6 in the nation's fifth largest city.

The city also will be unable to hold a $400 million bond election or a proposed tax limitation referendum without Justice Department approval.

The department's civil rights division ruled that Houston's 1977 and 1978 annexations of 14 tracts adding about 140,000 persons effectively reduced Houston's black population from 26 percent to 24.8 percent, and its Mexican-American population from 14 percent to 13.5 percent.

"Based on relevant court decisions, and in view of the relevant characteristics of the city of Houston, we find such reductions to be legally significant," Assistant Attorney General Drew Days III said.

The city has a population of about 1.7 million, including 680,000 minority members.

In a letter telephoned to the city legal department today, Days said the Justice Department would consider withdrawing its objections to the annexations if the city changed its voting system to include the election of "some" members of the city council from single-member districts, not at-large in the city.

The city council cannot make such changes alone; it would have to go to the city's voters for approval.

In a telephone press conference from the U.S. Conference of Mayors meeting in Pittsburgh, Houston Mayor Jim McConn said the city would attempt to compromise on city council composition with the Justice Department.

McConn said he and City Attorney Robert Collie might go to Washington next week to seek a clarification of the department's ruling.

He said he opposes appealing the ruling, but said a decision on such an appeal would not be made until after he meets with the city council Thursday.

All eight city council members are elected citywide, although five represent specific districts and must reside in those districts.

The Justice Department found that, although three of every eight Houstonians are either black or Mexican-American, only one black and no Mexican-Americans have ever served on the city council in its current form.

A suit by a coalition of liberal and minority groups challenging Houston's at-large city council system is before the 5th U.S. Circuit Court of Appeals.

Under the civil rights laws, Houston and other southern cities are required to submit annexations and other changes in voting boundaries to the Justice Department for approval.