Eight more Metrobuses have been withdrawn from service after hazardous cracks were found in their undercarriages, Metro officials said yesterday.

The action brought to 31 the number of buses removed from operation since last week. Metro officials have warned that the cracks -- found near the front axles of buses manufactured by Neoplan U.S.A. Corp., a West German-affiliated company -- could lead to loss of control over steering.

In another development, the National Highway Traffic Safety Administration disclosed what it described as "very disturbing" evidence of other safety-related defects in Neoplan buses used by Metro. The federal safety agency said that flaws in the buses' electrical circuits posed a fire hazard.

The agency's findings stemmed from a fire that demolished a Neoplan bus in Southeast Washington last December. Senior Metrobus officials were unaware of the federal agency's conclusions, a Metro spokesman said yesterday, and no action will be taken until Metro engineers have examined the issue.

Forty-four Neoplan buses remain in use by Metro, largely serving routes in Southeast Washington. Officials said the buses have undergone daily safety inspections.

After the Neoplan buses were withdrawn from service, Metro officials called in older buses from the transit system's reserve fleet as replacements. No interruption in bus service has occurred, officials said.

Officials for Neoplan U.S.A. Corp., a Colorado-based company affiliated with a West German bus manufacturer, Gottlob Auwaerter GMBH & Co., declined to comment yesterday. Last week, a Neoplan statement said the company did "not think that, at this point, a safety issue is involved."

Metro officials said that Neoplan initially offered to repair the cracks by reinforcing a section of the undercarriages. But the proposal was rejected, Metro officials said, because the suggested reinforcement would have concealed key components and prevented inspectors from determining whether potentially dangerous new cracks developed.

Hairline cracks were found in three Neoplan buses over the weekend and in five more buses yesterday, a Metro spokesman said. In all 31 buses, the cracks have occurred in components connecting the front axle assemblies with the main frames supporting the buses. The cracks have appeared in metal plates and tubes and in welds.

In a letter to Neoplan released yesterday, the National Highway Traffic Safety Administration said that an investigation of the Dec. 23 Metrobus fire indicated what appeared to be excessive amperage in a protective circuit breaker as well as absence of a cutoff switch described as a "standard safety feature."

A spokesman for the federal agency said that a circuit breaker with lower amperage might have prevented the fire. If the switch had been installed, damage to the bus from the fire might have been limited, the spokesman said.

The Neoplan buses have been a focus of controversy since they were purchased by Metro in June 1983 for $12.5 million. Last year, hairline cracks also were found in welds above the buses' rear axles.