The Metro transit authority maintains inadequate control over millions of dollars' worth of spare parts needed for the bus and subway system, according to an auditors' report issued yesterday.

The report, by Peat, Marwick, Mitchell & Co., found that the transit agency lacks safeguards to prevent possible theft or misuse of parts. The auditors also cited excessive supplies of unneeded parts and shortages of other parts needed frequently for repairing buses and trains.

Storage rooms where parts are kept have been left unlocked at night, the auditors said, and Metro employes have had "unlimited access" to inventories of parts kept in these rooms. "The laxity in controlling storeroom access has created an open storeroom situation and a lack of control," the report said.

The audit was ordered by Metro's board of directors partly as a result of rising costs for parts. Metro's inventory of spare parts currently amounts to $22.7 million. Total costs for parts and supplies have risen from $21.8 million in 1981 to $29.6 million this year.

The auditors warned that shortages of parts pose a "major problem" for Metro's maintenance staff. More than 10 percent of the 400 parts most frequently needed for bus and train repairs were out of stock, the auditors found.

"The stockouts reduce the productivity of the maintenance staff, since they are unable to complete work on a vehicle until the needed stocked-out parts can be obtained," the report said.

However, many parts stored by Metro appeared unneeded, the auditors said. They found that 36 percent of the items stored by the bus system and 67 percent of the items stored by the rail system had not been needed for at least eight months.

Metro has big enough inventories of some high-cost parts to last for several years, the auditors said. They urged the transit authority to reduce its parts inventory by $1 million during the next nine months, partly by selling off excess supplies. The move would save the agency $100,000 a year in interest payments and other carrying costs, the auditors said.

In a separate development, a committee of Metro's board of directors took a preliminary step yesterday that may lead to a test of the Reagan administration's policy toward financing expansion of the Metro subway system. The transit authority has planned eventually to enlarge the current 42-mile system to 101 miles. The Reagan administration has agreed to finance only about 76 miles.

The committee proposed asking the Urban Mass Transportation Administration to earmark part of the Metro system's $250 million federal appropriation this year for construction of tunnels and bridges outside the 76.4-mile network previously endorsed by the Reagan administration. The plan has not yet been reviewed by Metro's board, and it is unclear how federal officials will respond to the proposal.

"We're trying to make a case for going beyond the 76 miles ," said John S. Egbert, Metro's assistant general manager for design and construction.

Under the proposal, federal funds would be allocated, in part, for excavating subway tunnels north of Wheaton toward the proposed Glenmont station and on a portion of the planned Green Line near West Hyattsville. Funds would also be allocated for constructing bridges on the proposed Northern Virginia extension to the planned Franconia-Springfield station. None of these sections is included in the system that the administration has previously agreed to finance.