Metro buses are breaking down while in service almost twice as often as they did three years ago, transit officials reported yesterday.

They reported that an average of nearly 10 percent of Metro's buses could not be operated because of major mechanical problems each day during the last six months of 1980, although the transit officials said the problem has not become great enough to force the cancellation of any bus runs.

The figures released yesterday cap a steep three-year increase in bus breakdowns and maintenance problems.

"I'm afraid the situation is probably going to get worse before it gets better," said Theodore G. Weigle Jr., Metro's assistant general manager for transit services. He added that the problems occur with new buses as well as older ones.

"The new buses are built to lots of federal specifications and they're more difficult to maintain than the old ones," he said. "The old buses are aging, and we haven't been able to keep up the schedule of preventive maintenance."

Since 1978 the average distance between bus breakdowns requiring road service dropped from 3,167 to 1,690 miles. The proportion of buses kept out of service because they could not be operated rose from 6.8 percent to 9.7 percent in that period.

These figures were part of a string of discouraging statistics released at a Metro committee meeting yesterday. Metro officials said the figures show a worrisome slippage in the performance of the area's transit system as its fares and costs have soared.

For example:

Ridership on the subway virtually was unchanged compared to the last three months of 1979 even though three new stations had opened. Ridership on buses fell 5 percent. After the latest round of fare increases on Jan. 1, the number of subway passengers dropped 5 percent while bus ridership dropped 8 percent more.

Breakdowns on the subway also became more frequent at the same time as mechanical problems increased on the buses. Compared to a year earlier, the miles between breakdowns on the subway system decreased 7 percent.

Crime increased in subway stations and trains, with arrests up an average of 20 percent from a year earlier.

The reliability of Farecard machines declined after Metro employees were used to replace a private contractor, who had been criticized for not maintaining the equipment properly.

Fuel consumption on Metro buses continued to increase, with the average for all buses dropping to just 3.4 miles per gallon of diesel fuel.

"I think the public is still supportive of the transit system," said Cleatus Barnett, former Metro board chairman and head of the agency's revenue and operations committee. "But we have to be concerned that there's more slippage throughout the system than I would really like to see. The spread of the slippage is a problem."

"A lot of these numbers aren't as good as they ought to be," said Metro general manager Richard S. Page. "We're a large corporation, and much of what's happening to Metro is similar to other problems in our society. . . We're trying hard to do better."

Officials of the American Public Transit Association said bus systems throughout the country have been reporting an increase in maintenance problems, particularly because of heavy lifts required by federal rules to provide access for the handicapped.

"The bus system here needs a lot of attention," said Weigle, the assistant general manager. "It's not as glamorous as the rail system, and it just got less attention than the subway. People have seemed to let it limp along. But we're committed to doing more now."