The U.S. space program was staggered by another setback yesterday when engineers discovered that a second space shuttle has a leak in a critical liquid hydrogen fuel line, forcing NASA to impose an indefinite halt in future manned flights.

While a team was still being formed to investigate a major flaw in the orbiting Hubble Space Telescope, engineers at Kennedy Space Center in Florida discovered for the second time in a month that a shuttle with a major defect had passed a gantlet of supposedly rigorous tests and inspections, making it all the way to the launch pad.

Yesterday's leak in the shuttle Atlantis was in the same area as the massive leak a month ago that forced postponement of the shuttle Columbia's astronomy mission six hours before it was to be launched.

The newest leak was found in a special test carried out after engineers were unable to pinpoint or explain the cause of the first leak, and when questions were raised about some of the tests the first leaky components had passed. The test consisted of loading some of the shuttle's fuel and passing it through the fuel line from the huge external tank to the engines in the orbiter. Normally this step would not have occurred until just hours before launch when the fuel was loaded.

Like Columbia, Atlantis will have to be rolled back to its hangar for the problem to be fixed, William Lenoir, chief of spaceflight for the National Aeronautics and Space Administration, said at a briefing.

"There's no question that we won't fly until we understand it, have it fixed, have tested it, verified it and we're ready," Lenoir said. That could take anywhere from two weeks to much longer, depending on what is causing the leaks and whether parts must be redesigned.

The site of the leak in both shuttles -- pipes connecting fuel tank to orbiter -- were part of a batch of eight tested at about the same time in 1984. All eight leaked during initial tests at a California contractor facility, but conditions of the test were changed so that the hardware could pass.

Of those eight, the two now installed in Columbia and Atlantis showed the greatest leakage during initial tests, according to a NASA source.

Officials said all tests and manufacturing procedures will be reviewed.

Before the Atlantis is rolled back, Lenoir said, technicians will "troubleshoot" the leak area in hopes of finding the hole, in case the leak cannot be reproduced later. After numerous tests, engineers are still unable to pinpoint or explain the earlier leak on Columbia.

The fuel connector system is considered one of the most dangerous on the shuttle and was redesigned for added safety following the 1986 Challenger disaster. Officials say batteries of sensors on the launch pad would prevent a shuttle from launching with an unacceptable leak. The system consists of two halves, a pipe and valve connected to the orbiter which is flown repeatedly, plus a similar pipe and valve connected to the external tank, which is jettisoned after each launch and is replaced.

In initial tests, at subcontractor Parker Hannifin in Irvine, Calif., the tank halves leaked when attached to a "slave unit" to simulate the orbiter half. But when the slave unit was replaced with a flat plate, there were no leaks, leading engineers to conclude that the leaks had been caused by the ground test equipment and not by flight hardware.

In addition, instead of using explosive liquid hydrogen in the tests, the safer liquid nitrogen was used, but it is 100 degrees warmer and may not cause the metal pipes to shrink as much as the real fuel.

Although engineers said they had analyzed the tests carefully and believed they had performed a valid test, some voiced concerns in light of the Columbia leak that these and other tests should be reviewed.