The General Services Administration this week announced a six-week extension of the deadline for the environmental study of the proposed John F. Kennedy Memorial Library, and declared its confidence in C.E. Maguire Inc. to prepare a competent impact report.

The announcements made Thursday by GSA Administrator Arthur F. Sampson ended more than a month of uncertainty over the preparation and review of the environmental study.

GSA decided to retain Maguire to prepare the report, and has rescheduled a public hearing on the draft statement for September 16.

Under the original schedule, Maguire's draft report was due midJune and a public hearing on the study set for late July, when many Cambridge residents are on vacation.

Both the Harvard Square Development Task Force and Maguire had, for different reasons, requested a delay in the study. The task force claimed the old schedule would not provide Cambridge residents with sufficient opportunity to comment on the statement's findings.

George Grant, Maguire's director of the study, last month asked the GSA to extend the 120-day June deadline because of the time lost establishing relations with the Cambridge community and problems created by the lack of definite library plans. Architect I.M. Pei is scaling down the library plans in response to increased construction costs and community criticism of the $27-million project.

In his statement, GSA's Sampson said the new schedule "will ensure a full hearing from community interests."

Shortly after Maguire's appointment on February 13, Cambridge civic leaders asked the GSA to investigate several allegations against the firm, including the charge that Maguire had unethically altered a report under pressure from another client, the Massachusetts Port Authority.

Sampson said the departmental review concluded the charges are "unfounded," but he declined to issue a copy of the report on Maguire.