The British army is knowingly flying up to 200 light helicopters containing "suspect" parts from an American supplier, it was learned today.

A Defense Ministry spokesman acknowledged that components in the Sioux helicopters have been falsely labeled. He insisted that "none of these components affect flight safety' and therefore the vehicle has not been grounded.

The Sioux's British maker, however, disclosed that the bogus part is the drive assembly supplying power to the tail rotor.

"If they failed," he said, "it could have a serious effect on the flight safety of the helicopter."

The Sioux, which is used by British forces in Northern Ireland, needs two rotors to stay aloft. Failure in one would probably bring down the craft with its two-man crew.

The Washington Post also learned today they the West German government is investigating the alleged bogus helicopter parts scheme and that in 1971 the German Defense Ministry suspended direct business dealings with Aviation Sales Corp., a New York manufacturer that allegedly has sold millions of dollars worth of bogus parts.

According to West German government spokesman in Bonn and Washington, the action was taken after the Defense Ministry bought replacement parts for the F-104 jet fighter from Aviation Sales through a West German company called Flugzug Union. The parts turned out not to be authentic, name-brand parts that they purported to be.

According to figures supplied recently by the Lockheed Aircraft Corp. in Los Angeles, there have been 178 crashes involving F-104s made in West Germany and flown by West German pilots. Many of the crashes have never been satisfactorily explained, West German officials said.

The principal source for the information about Britain's Sioux helicopters was John Teague, public relations executive for Westland Aircraft in Yeovil. Westland is the sole maker of the Sioux for Britain and, through the Ministry of Defense, sells them to foreign armies.

The Yeovil company bilds the Sioux under a license from its designer, the American Bell Helicopter co. The "bulk" of the bogus parts, Teague said, were supplied by Aviation Sales Corp.

Teague said that Bell and Westland inspectors, alerted since earily January, discovered that the identification numbers on the tail rotor's drive assembly had been "altered." In effect, he said the altered number falsely identified the part as meeting new and staffer specifications to give it a longer life.

He insisted that the fake numbers had been applied to a component that was genuine. The change, he explained, improperly upgraded the piece.

Teague acknowledge, however, that Westland has conducted only an "elementary" test on the part he called "suspect."

The company has simply cut into the drive assembly to see if it has corroded. Westland has not yet subjected the piece to tests of its ability to withstand heat, strees and fatigue, according to Teague. He though these test might begin Monday.

Teague insisted that there was no need to ground the helicopters or remove the suspect parts because they had a usable life.

"We don't see this as a critical emergency," he said. "Why should we assume they [the drive assemblies] are going to fail? It's an acceptable level of uncertainly. The likely life is many years. It was a usable part.'

How long the tampered parts are usable Teague could not say. The "elementary" tests, he acknowledged, do not supply that information.

Moreover, th Sioux contain other parts produced by Aviation Sales or its subsidiaries, Teague said. Westland has not yet had time to examine them.

In Los Angeles, Bell, the Sioux designer, announced that Westland and the Defense Ministry "are taking immediate action . . . to remove these [bogus] parts from service." But Westland and the Defense Ministry say they are doing no such thing.

Bell, said Teague, "have a commercial interest, of course." He implied that the U.S. company wanted the bogus parts removed only so it coulc sel the necessary replacements.

"We will do nothing until we make a cool analysis," the Westland executive said.

His company's dismissal of danger has apparently been accepted by the Defense Ministry. As far as is known, it has made no independent checks, but is relying on Westland.

Teague estimated that the army is now flying about 200 Sioux and a "significan tnumber" contain the bogus drive assemblies. He did not know which foreign armies were also employing the model or how many they are flying.

A senior West German official in Washington said that the investigation into possible bogus parts in West German military helicopters is under way, both in the United States and West Germany. "Unfortunately," said the offical, "Aviation Sales also supplies helicopter parts to the West German military through Westland.'