Federal courts throughout the country will begin no new jury trials in civil cases starting Monday because Congress has not given final approval to a $3.8 million supplemental appropriation, the U.S. Judicial Conference announced late yesterday.

The judges group said the action, was necessary because funds earmarked for juries in this year's federal budget are running short. Congress reduced the courts' request for jury funds by $2.8 million. An additional $1.8 million was trimmed by the Gramm-Rudman-Hollings deficit-reduction act, bringing the fund to $41.2 million.

Nationwide, the cutoff could affect about 1,200 civil cases that normally use juries from mid-June until Sept. 30, the end of the current fiscal year. But juries will continue to be available for crimi- nal cases, and all civil jury cases already under way will be completed.

The judicial group, which is headed by Chief Justice Warren E. Burger, said when it made its budget request in March that it might suspend civil jury trials this summer.

The funds have been approved by both houses of Congress as part of an overall supplemental budget package, which went before a House-Senate conference committee yesterday. The committee is expected to finish action next week.