"NASA managers met Friday afternoon and determined space shuttle Endeavour will launch no earlier than Monday, May 16 at 8:56 am," the space agency said.

Technicians will continue to repair and retest the electric circuitry that caused NASA to postpone the launch on April 29 less than three hours before liftoff, it said.

The May 16 launch is supposed to be the US space program's second to last shuttle flight to the International Space Station, followed by Atlantis in June. After that, the 30 year old shuttle program will end.

The glitch has been traced to a power problem in the aft load control assembly-2 (ALCA-2), a box of switches that control electrical flow to heaters that keep fuel lines from freezing in orbit.

The piece was replaced on Wednesday. Since then, technicians have been running a battery of tests on the box and will continue to do so over the weekend.

NASA has scheduled a press conference Monday at 3 pm (19H00 GMT) to provide an update.

The six astronauts, including Italian Roberto Vittori of the European Space Agency, returned to their home base in Houston, Texas early Sunday to continue preparations for the launch.

Endeavour will carry a $2-billion, seven-ton particle physics detector, the Alpha Magnetic Spectrometer-2, which will be left at the space station to scour the universe for dark matter and antimatter.

The 14-day mission, known as STS-134, is to be commanded by US astronaut Mark Kelly, whose wife, US congresswoman Gabrielle Giffords of Arizona, is recovering from a gunshot wound to the head sustained in January.

Giffords was allowed by her rehab doctors in Houston to fly to Florida to watch the planned April 29 launch, and she is expected to return again for the next attempt, her office said.