CAPE CANAVERAL, Florida — NASA on Sunday delayed the final launch of the space shuttle Endeavour until the end of the week at the earliest, after technical problems uncovered last week proved more complex than originally thought.

In a new delay for the US space program's penultimate shuttle launch after last week's cancelled flight, NASA said electrical failures in the power supply to a fuel-line heating unit had caused engineers to call off the launch hours before liftoff on Sunday.

The US space agency said a plan to try again on Monday now also is a no-go.

"Based on the amount of time needed to do the fix, a new shuttle launch attempt will not happen before the end of the week, at the soonest," NASA said.

A press conference to discuss the problem is expected at 1800 GMT on Sunday.

Endeavour's mission to the International Space Station will mark the second to last shuttle flight ever for the US program, which will end for good later this year.

Endeavour is poised to carry a potent, multibillion dollar tool for searching the universe in the second-to-last flight for NASA's 30-year program.

After Endeavour returns from its 14-day mission and Atlantis launches for a final time in June, the space shuttle program will close for good.

That will leave Russia as the sole taxi for astronauts heading to and from the orbiting space lab until a new spaceship is built by a partnership between NASA and private companies, by 2015 at the earliest.

Officials said electrical problems were the likely culprit in last week's cancelled launch.

"Technicians and engineers are narrowing in on the likely source of what caused heaters on a fuel line for space shuttle Endeavour's auxiliary power unit-1 (APU-1) to fail on Friday," NASA said.

"Teams will meet Monday and are expected to determine a new 'no earlier than' next launch attempt for Endeavour at that time," the US space agency said in a statement.

"There still are numerous factors to be worked out, but just based on the amount of time needed to do the fix, a new launch attempt wouldn?t happen before the end of the week, at the soonest."

The crew of six astronauts was heading back to Johnson Space Center in Houston "for a few days of additional training" before returning for the next launch attempt, NASA said.

President Barack Obama, who had planned to watch the launch Friday before it was cancelled, flew in anyway and chatted with the six astronauts at Kennedy Space Center, the White House said.

The president also met with wounded lawmaker Gabrielle Giffords, who moved to Florida from a rehabilitation facility in Texas for the launch.

Giffords was shot in the head at a neighborhood political gathering in January but was allowed by her doctors in Houston, Texas to take a break from rehab in order to watch her husband, Mark Kelly, command the shuttle.

The shuttle Endeavour is the youngest of the three-member space flying fleet. It was built in the wake of the Challenger disaster in 1986 and flew its first mission to space in 1991.

Discovery, the oldest, flew its last mission in February and March, and is in the process of being stripped of all its valuable components ahead of its retirement in a museum on the edge of the US capital Washington.

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 clues as to how it all began.

The final shuttle mission, Atlantis' 12-day voyage to the ISS set for late June into early July, will bring the storied program to its final end, space officials said.

There have been six space shuttles in all, including Endeavour, Atlantis and Discovery.

Enterprise was a prototype that never flew in space; Challenger exploded after liftoff in 1986, killing all seven on board; and Columbia disintegrated on its return to Earth in 2003, also killing seven astronauts.