NASA yesterday postponed the space shuttle's return to flight, scheduled for June 2, after engineers discovered that a part in the redesigned shuttle booster had failed during a test firing last week in Utah.

The extent of the delay will not be known until the engineers can take apart the rocket motor and figure out what caused the failure, officials of the National Aeronautics and Space Administration said.

Booster hardware to be used on the next flight of the shuttle Discovery -- the first since the Challenger exploded on Jan. 28, 1986 -- was to be shipped from Utah to Kennedy Space Center in Florida by this weekend for assembly. Now it will be held in place until the engineers determine whether it will have to be rebuilt.

NASA engineers and Morton Thiokol, which manufactures the shuttle boosters, initially had declared last Wednesday's cold weather test firing at Morton Thiokol's plant near Brigham City, Utah, an apparent success.

However, more detailed examination revealed that a large section was missing from the ring that anchors the booster's nozzle, the cone-shaped section at the rear that steers the giant rocket, to a flexible "boot" that allows the nozzle to swivel.

The piece went around approximately 140 degrees of the circumference and was 9 inches wide. One purpose of the ring -- called the "outer boot ring" -- is to shield the metal and rubber bearing at the core of the swivel mechanism from hot gases. Pieces of the missing ring were found inside the motor.

The failure "surprised us," John Thomas, head of the NASA booster redesign team, said yesterday.

Since the Challenger exploded 23 months ago, killing the crew of seven and grounding the remaining three shuttles, the NASA-Thiokol team has redesigned many elements of the boosters. The new design for the field joints, which connect segments of the 126-foot-long rockets and were found to have caused the Challenger disaster, appears to have functioned perfectly during last week's test, officials said.

That test was the second of four full-scale, full-duration test firings of the boosters planned before the next flight, but it was the first that incorporated all of the designs to be used in actual flight. Although they are on a tight schedule with no margin for error, NASA officials had until yesterday expressed increasing optimism about making a June shuttle launch.

The 8-foot diameter outer boot ring was among the elements that have been redesigned because of earlier problems. It is constructed of a special carbon cloth laid on by hand in many layers and then coated with phenolic, a substance like epoxy that hardens it. Designed for exposure to the intense heat of the rocket gases, it is supposed to erode away in a measured fashion.

In earlier flights, small bits of the ring had been found to have chipped away. This was not considered serious but could have signaled a potentially greater problem, because if too much is lost too fast, the whole ring begins to split apart, allowing hot gases to reach more vulnerable parts, specialists said. Depending on when this happened during flight, the ultimate effects could be loss of steering and rupture of the motor, according to NASA systems engineering official David Winterhalter.

Engineers had changed the method for layering the carbon cloth and tested it for the first time last Wednesday. If it is determined that the new design is the problem, officials said, engineers have a "fall-back" interim design that was tested successfully last fall.

"We have no idea what caused the failure right now, and it will be a number of days before we can find out," Winterhalter said. Analyzing the problem will be delayed by the lingering heat and hot gases that prevent engineers from taking the engine apart right away, he said.

"If we have to rebuild the flight hardware," Thomas said, "I expect the delay will be measured in weeks."

"From what little we've heard, this could be a serious problem, both for the schedule and for the design," said Myron Uman, project director of a panel of the National Research Council, which has been monitoring the booster redesign. "But we really need more information."