The government's award of a 20-year NASA office lease valued at more than $350 million to publisher Mortimer B. Zuckerman's real estate company was improper, a congressional watchdog agency said yesterday.

The General Accounting Office recommended that the government reimburse one of the losing bidders for the money his partnership spent competing for the contract and appealing the award. That would total millions of dollars, according to the bidder, developer Peter Schwartz. The GAO said the government nonetheless should stand by the lease it signed with Zuckerman's Boston Properties this year.

The agency found "no evidence that Boston Properties engaged in any improper activity, or was aware of any impropriety."

GAO reserved its criticism for the General Services Administration (GSA), which procures office space for the government. GSA officials gave Schwartz "misleading and improper" guidance about the rules of the competition for the National Aeronautics and Space Administration lease, GAO said.

Robert C. MacKichan, GSA general counsel, declined to comment on the GAO opinion last night, saying the agency has not had time to study it. Schwartz said he was still reviewing his options. GSA has 120 days to follow GAO's recommendation or reject it and explain its reasons to Congress.

Boston Properties's proposal calls for construction of the new NASA headquarters on the site bounded by the Southwest Freeway and Third, Fourth and E streets SW. Peter N.G. Schwartz Cos.'s Judiciary Square building is already standing and vacant in downtown Washington.