The 20th mission of the space shuttle was postponed again today when a guidance computer aboard Discovery failed 25 minutes before liftoff.

Discovery now cannot get off the ground until at least Tuesday, NASA said. On Saturday, bad weather halted the first attempt to launch the spacecraft.

The delay complicates the crew's chances of catching and reviving a dead Navy satellite drifting in space. The last day that Discovery can leave Earth on this mission and attempt to salvage the satellite is Thursday.

If the shuttle is not launched by Thursday, its crew of five will carry three communications satellites into orbit and return to Earth without trying the salvage mission for which they have trained during the last four months.

The drifting satellite, insured for $85 million, is called Leasat and has shown no life since being placed in orbit by a previous shuttle crew on April 13. Hughes Communications Corp. is paying the National Aeronautics and Space Administration $8.5 million to attempt to revive Leasat, which Hughes had planned to lease to the Navy.

Discovery's launch is scheduled for between 7:02 a.m. and 7:45 a.m. EDT Tuesday.

"We have to delay for two days because we have to inspect the machinery in our engines, which have gone through two thermal cycles of tanking and detanking some very cold fuels," Launch Operations Director Robert Sieck said at the Kennedy Space Center. "The liquid hydrogen that moves into the engines is so cold more than 420 degrees below zero it could cause manifolds and ducts to bend or even buckle, which could prove catastrophic."

The two consecutive launch delays have cast new doubts on the shuttle's ability to meet a tight schedule. The spacecraft's sensitivity to weather and what appears to be a tendency to suffer single-part failures like the one that delayed liftoff today are of rising concern to shuttle managers trying to get their customers into space on time.

Each day lost costs the space agency at least $1.5 million and pushes back preparations for future flights. Atlantis, the newest member of the shuttle fleet, had been scheduled to move to the launch pad Tuesday.

Today's setback marked the fifth time that one of the shuttle's computers has failed and the second time that one has failed in the closing minutes of a countdown. Computers have failed three times in flight, but have not imperiled or cut short a mission.

The shuttles' computers were developed in 1972. NASA has contracted with IBM to develop a faster and more reliable computer, which is expected to be ready in 1987. The new model is said to be three times faster and have three times the memory of the current flight computers.

Today's balky computer, one of five on board, was acting as the spaceship's backup computer. The backup computer is supposed to take over if the other four experience a failure that could destroy the shuttle in the crucial first 10 minutes of its ascent.

"This is why we scrubbed today's launch," Shuttle Program Manager Arnold D. Aldrich said. "It is disconnected from the other four and takes care of everything if there is a sudden generic failure that incapacitates the other four."