Space agency officials confirmed yesterday the existence of a significant and persistent fuel leak in the umbilical line that connects at least two of the nation's space shuttles to their silo-shaped fuel tanks.

Discovery of the still mysterious and potentially explosive leak was responsible for indefinitely grounding the entire shuttle fleet two weeks ago after hydrogen gas was detected while the Atlantis and Columbia spacecraft were on the launch pad.

Weekend tests at a Rockwell

International laboratory convinced engineers that the fuel leak aboard the two shuttles is serious and

persistent. It occurs when super-cold liquid hydrogen fuel is pumped through the pipes, flanges and

seals that form the umbilical connecting the winged orbiter and its fuel tank.

The tests at Rockwell used the umbilical cord from the shuttle Columbia. Atlantis awaits another round of tests on the launch pad at Cape Canaveral.

Officials of the National Aeronautics and Space Administration said the leak probably came from a hole no bigger than the diameter of a pin, though it is still possible there are additional leaks.

"We really don't know what the problem is yet," said Robert Crippen, astronaut director of the space shuttle program. Crippen, however, said he was happy that Rockwell engineers had detected a leak in the laboratory and said he was confident its exact location would be found shortly.

By Saturday, NASA engineers plan to pump liquid hydrogen through the umbilical cord on the shuttle Atlantis, which sits on the launch pad in Florida bristling with sensors and detectors that workers hope will pinpoint the leak.

The main suspect is a Teflon seal used in 17-inch diameter pipe in the umbilical assemblage. The pipe brings liquid hydrogen from the tank to the orbiter's main engines. After liftoff, the external fuel tank falls away and the umbilical cord is severed.

NASA officials said it now

appears "extremely unlikely" that

the agency will be able to complete two shuttle missions as scheduled before the launch of the Ulysses

solar probe, which must be deployed from the shuttle before its window of opportunity closes in October.

Instead, NASA might fly a secret Defense Department mission and defer again an already twice-delayed scientific mission called the Astro observatory, which contains ultraviolet and x-ray telescopes and sensors.

In other developments, NASA yesterday grounded two veteran shuttle crew commanders. Navy Cdr. Robert L. "Hoot" Gibson, involved in a fatal midair collision of stunt planes in Texas on Saturday, was grounded for one year for

violating a policy that restricts

the high-risk recreational activities of astronauts assigned to flight crews. Gibson's plane collided with another stunt plane, sending it hurtling to the ground and killing its pilot.

The other grounded astronaut, Navy Capt. David M. Walker, was replaced on an upcoming shuttle mission as the result of a May 1989 incident in which Walker was piloting a jet reported to have nearly collided with a Pan American Airways flight.