Three bogus replacement parts installed on some U.S. jetliners have been identified as potentially hazardous, and airlines must remove them immediately or adopt special procedures to prevent mishaps, the Federal Aviation Administration said yesterday.

At the same time, the FAA identified a third Seattle, Wash., area business as a source of parts never certified as safe but sold to U.S. airlines for use on Boeing 737 jets.

The FAA said that immediate action was ordered on the three bogus replacement parts after Boeing and FAA engineers found poor workmanship, low-quality materials and inadequate safety standards. Moreover, analysis showed serious consequences in the event the parts malfunctioned. The three parts involved two systems:

A system in which wing spoilers pop up automatically and slow a 737 as soon as the wheels touch down on landing. The FAA said that if the spoilers flipped up during flight, the plane could not fly.

An engine fire detection and control system. If a pilot was alerted to a fire in one engine and acted to put in out, the FAA said the other engine might be turned off. Boeing 737s have only two engines.

FAA said airlines must remove the bogus parts immediately or adopt the special measures - directing pilots not to use the suspected spoiler system automatically and testing the fire devices daily. The agency emphasized that no failures or malfunctions of the parts have been reported.

The FAA was unable to say how many 737s are fitted with the three types of bogus parts but said it was a "small number" of the approximately 190 737s in service in the United States.

Yesterday's disclosure by the FAA marked a retreat from the original announcement of trafficking in 28 different bogus Boeing 737 and 727 parts, in which the agency said there was "no evidence that an immediate hazard exists." Last week, the agency gave the victimized airlines 30 days to remove some parts and 45 to remove the rest.

The three singled out yesterday were in the group scheduled to be removed within 30 days (by March 18), if not removed immediately. That deadline still holds for the three potentially trouble-making 737 parts.

The agency identified the third source of Boeing 737 parts not certified as safe as Air Spares International Inc. of Seattle. It had earlier named ADS Supply Co. and Air Repair Inc. The FAA said the companies marketedthe replacement parts with bogus Boeing labels and serial numbers, leading their airline customers to believe the parts had been certified by FAA representatives as safe for use.

When the FAA announced the alleged scheme,it estimated that up to 100 planes at five airlines could be involved, but it could not be determined yesterday whether the addition of Air Spares International raised that possible total.