The Senate, anxious to avert a filibuster by pro-busing forces, yesterday defeated legislation intended to limit the authority of federal judges to order busing in school desegregation cases.

By 49 to 47, the Senate tabled and thus killed a measure that would have forbidden federal judges from ordering busing unless there was evidence of international discrimination.

Offered by Delaware's two senators, the amendment to a $55 billion bill to extend federal elementary and secondary education programs also would have stayed all pending busing orders, including a cross-district plan scheduled to go into effect Sept. 13 in Wilmington, Del.

Both sides called the amendment the most far-reaching antibusing measure to receive serious consideration in the Senate, but opponents also said it probably was unconstitutional. The reason is that the courts generally have based their busing orders on the Constitution, which has primary over an act of Congress.

Sen. William V. Roth Jr. (R-Del.) said the amendment attempted to set guidelines for judges considering desegregation cases. He said it would require a court to find that "a discriminatory purpose in education was a principal motivating factor in the constitutional violation for which busing is proposed as a remedy."

But it was attacked by Sens. Edward W. Brooke (R-Mass.) and Robert B. Morgan (D-N.C.) as a move calculated to disrupt existing busing programs and breach the separation of powers between the legislative and judicial branches.

Claiming he would introduce 95 stallling amendments to the education bill if the antibusing measure passed, Brooke said the proposal was blatantly unconstitutional and would generate "widespread public confusion."

Morgan said the amendment would create a double standard between the South, where schools were segregated by law, and the North, where they are segregated by housing patterns and real estate practices.

"Busing in the South is working," said Morgan, who opposed busing orders when they came to his state. "It's not desirable, but we're doing it. It's now a fact and we're doing it all over the South."

The deciding voice in the vote, however, came from Majority Leader Robert C. Byrd (D-W. Va.), who said he supported the concept of the amendment, but urged senators to vote against it because it would tie up the Senate at a time when members are eager to get home for a Labor Day recess, scheduled to begin tomorrow.

After the vote, Sen. Joseph R. Biden Jr. (D-Del.), the amendment's other chief cosponsor, said the close vote signals "the death toll" for supporters of busing in the Senate. "I really think the significance of the vote is that time is getting close" for an end to busing, he said.

The amendment was on a bill to extend the Elementary and Secondary Education Act of 1965, the cornerstone of federal aid to education programs.

The Senate version of the bill would extend all ESEA programs, aimed at aiding disadvantaged students, at a cost of $55 billion by 1983. It also would provide, for the first time, aid to children attending private schools.

The House passed a less expensive version of the bill July 13.

The Senate version includes extending the controversial impact aid program at about current levels - estimated at about $1.4 billion next year. President Carter had wanted to trim the program by some $76 million to $735 million a year.

The idea of impact aid is to offset the burden placed on school districts by government employes who work at tax-exempt federal installations and military bases. It is a major factor in school budgets in the District of Columbia and the Virginia and Maryland suburbs.

On the busing amendment, only one area senator, Charles Mc C. Mathias Jr. (R-Md.), voted to table the measure.