The U.S. Court of Appeals delayed yesterday until at least Sept. 28 the implementation of a court order that would block the transfer of most newly sentenced D.C. inmates to the city's Lorton Reformatory.

The earlier order, issued two weeks ago by U.S. District Judge June L. Green, would have sent D.C. inmates to federal prisons.

City officials said the appeals court's move imperils their efforts to meet court-ordered inmate population limits at Lorton's Central and Occoquan facilities.

The appeals court granted the federal government's request for a stay while the order is appealed, declaring that the federal government had "demonstrated a substantial possibility of success on the merits of the case."

At the same time, the three-judge panel turned down the federal government's request for an immediate reversal of the order and set a Sept. 25 hearing on the matter. The federal government had estimated that it would be required to take 300 to 500 D.C. prisoners each month if the order is put into effect.

Green, citing chronic crowding at the city's prisons, handed down the order blocking the transfer of additional inmates on July 30. It was to have gone into effect last Wednesday, but was delayed temporarily while the court considered the longer stay.

Green's order came two weeks after she reversed her seven-year-old ruling in the case and added the attorney general as a defendant. She ruled that the facilities at Lorton no longer were suitable to house additional inmates and ordered the attorney general to take inmates into federal facilities.

The federal government appealed the order, saying that Green did not have the legal authority to add the attorney general at this time. Even if she had the authority, the government argued, she had "overreached" in closing all of Lorton to new prisoners because the suit before her dealt only with crowding at one facility, Central, a medium-security institution.

Green has ordered the city to reduce by 100 the number of inmates at the three Occoquan facilities by the end of this month. At the July 30 hearing, Green found the city in contempt of court for exceeding the population limits at Central and imposed a $250 a day fine for each of Central's dormitories that exceed court-ordered capacity limits.

A prison official said 13 dormitories remained over capacity yesterday, but there was no official report on the current amount of the fine.

D.C. Corporation Counsel Frederick D. Cooke Jr. said the city will appeal Green's orders regarding the Central and Occoquan facilities.

"We are going to have to rely on the Emergency Powers Overcrowding Act and on our efforts to try to place people in halfway houses," Cooke said. "And we are also continuing our negotiations with the prisoners' lawyers to increase the caps on facilities at Occoquan."

Green ordered the city to meet with prisoners' lawyers in an attempt to raise the current capacities at Central, Maximum and the three Occoquan units.

D.C. Corrections Director Hallem H. Williams Jr. said the delay ordered by the appeals court was "particularly troubling" because the city had relied on the federal government's taking inmates to meet Green's order to reduce the Occoquan population.

"We don't have the latitude" to meet the court order without aid from the federal government, Williams said.

The population of the city's prisons has risen by about 200 inmates a month since the first of this year. However, population figures filed in U.S. District Court earlier this week in connection with a separate lawsuit showed that the overall population of city prisons peaked at 7,958 the week of Green's ruling and had dropped to 7,817 by Monday.

During the same period, the overall population of the Occoquan facilities dropped from 1,957 to 1,907. Green's ordered population limit at those facilities is 1,281.

Corrections spokesman Edward D. Sargent said a total of 200 inmates were released under the emergency prison overcrowding act, which permits the early release of inmates convicted of nonviolent crimes, and through regular paroles during the last two weeks in July.