The National Transportation Safety Board (NTSB), which certifies the US-manufactured jet, wrote to aviation authorities urging them to replace the suspect system and arrange special training for 737 pilots in order to react appropriately to uncommanded sudden movements of the plane's rudder.
The board stopped short of grounding the jet, or declaring it unsafe. Investigators noted that the failure of the valve is rare, and recent changes in operating procedures by airlines could compensate for the problem.
The jet's rudder system has sparked a controversial debate in the airline industry. Safety officials believe that two crashes in the United States in the last five years, which claimed 155 lives, were caused by rudder malfunctions. In both crashes, the plane was nearing the airport in clear weather and suddenly, inexplicably rolled out of control and plunged to the ground.
Because of the jet's high-precision design, sudden rudder movements can cause a plane to swerve and roll almost simultaneously. At low speed and low altitude, a rudder pushed to an extreme position could put a 737 into a nosedive in seconds.
The board's letter also asserted that the 737's rudder system, first developed 30 years ago, would not meet today's standards for certification as "airworthy".
Boeing disagreed with US safety officials, saying that the "737 has one of the best safety records of any aircraft".
Last month, Vice President Al Gore said that the Federal Aviation Authority, the US aviation regulator, planned to propose replacing the rudder's valves. That proposal has not been made - because Boeing is still designing the replacement valve.
Safety officials took the unprecedented step of issuing a 10-page letter because of the delay in fitting the new rudder systems. Experts say that it would take two years to certify and install the new parts.
David Learmount, safety editor of Flight International magazine, said the board's action was "remarkable". "It is a clear message to the aircraft world to get their skates on. After all once you design the new system, you have to certify it. Then you have to produce it and then you have to install it. The NTSB obviously wanted things to be speeded up," he said.
It would cost more than pounds 80m to replace the vital parts of the rudder systems on the world's 2,800 jets..
Boeing says it is already revising pilot programs to account for "unusual" movements. It defines these as those in which the nose rises sharply by 20 degrees, drops suddenly by 10 degrees or the aircraft banks violently by more than 40 degrees.
British Airways, which operates a fleet of more than 60 737s, said it would not put passengers' lives at risk. "We have made precautionary checks on the planes which have turned up nothing," said a spokesman.
The Civil Aviation Authority, which regulates British aircraft safety, said it was awaiting the US response before acting. The authority is also devising a pilot's training program to cope with uncommanded rudder movements.Reuse content