New tests have revealed design flaws in the B1 bomber's electronic warfare system that could prevent the warplane from ever achieving the full capabilities the Air Force planned for and force the Pentagon to reroute possible bombing runs through the Soviet Union in the event of nuclear war, according to an internal congressional document and Pentagon sources.

The tests found that parts of the plane's electronic countermeasures system - designed to help protect the low-flying, penetrating aircraft on its most critical bombing missions - do not work and cannot be corrected without major changes, Air Force officials said."The existing system was discovered to be incapable of meeting the . . . specification requirements," the Air Force said in a statement. "For that reason we are now assessing our options."

House Armed Services Committee Chairman Les Aspin, D-Wis., who received a classified Air Force briefing late last week on the problems, called the test results a major setback for the $28 billion bomber program.

"The plane was designed to be capable of a nuclear attack against the Soviet Union," said Aspin, a critic of the Air Force's management of the controversial B1 program. "It is now called into question whether it will ever be able to do that mission in the way it was advertised."

Air Force officials argued that the bomber can perform its mission but said the limitations could force planners to "compensate for that through some other aspect." Although the nature of possible changes is classified, officials said these could include changing planned bombing routes, relying more heavily on other capabilities of the aircraft and using other weapons systems against Soviet defenses.

The B1 bomber has been one of the most controversial weapons systems in recent history. The Pentagon began production of the B1, but President Jimmy Carter killed the program, saying it was too costly and was made unnecessary by development of "Stealth" bombers designed to make detection difficult.

President Reagan revived the B1, calling it a critical leg of his strategic triad, and won congressional funding for 100 planes by vowing to keep a ceiling on the price. Reagan used the plane's sophisticated electronic jamming and receiving system - the brains of the bomber - as a major selling point.

"There's a tremendous capability still there," said one source familiar with the problems. "But it may not be everything the Air Force originally wanted."

Aspin argued the problems "increase risks for the pilot and diminish the capability of what we bought."

Since the first of 100 bombers became operational in the fall of 1986, the plane has been plagued by problems ranging from its flight controls to its terrain-hugging radar.

The recently completed tests, carried out by the Air Force and Eaton, the ECM system's builder, revealed "limitations within the current system architecture" that will "prevent the B1 ECM from ever achieving full operational capability," Aspin wrote in a memorandum distributed to subcommittee members this weekend.