• Boeing's first Starliner flight with astronauts on board was called off in the final minutes on Saturday.
• Boeing, NASA and the United Launch Alliance also canceled the backup launch date scheduled for Sunday. The next possible launch dates are June 5 and June 6.
• Starliner's crew debut has already been delayed by years. SpaceX's competing Dragon capsule has flown astronauts for NASA regularly since 2020.

Boeing's first Starliner flight with astronauts on board was called off in the final minutes Saturday, and the backup Sunday launch date was also canceled.

The company was targeting a June 1 launch at 12:25 p.m. ET of its capsule, which would have carried astronauts to the International Space Station for the first time in a final major test of the system.

Leaders from Boeing, NASA and the United Launch Alliance, or ULA, held a press conference later Saturday to provide updates on the malfunction and the status of the next launch attempt.

"The disappointment lasts for about three seconds," said Mark Nappi, vice president and program manager of Boeing's Commercial Crew Program. "And then you just immediately get busy and do your job."

Boeing had a backup launch date scheduled for Sunday at 12:03 p.m. ET. But Saturday evening, NASA announced the cancellation of that Sunday launch "to give the team additional time to assess a ground support equipment issue" at the Florida launch site.

NASA said it will provide further updates. The next possible launch dates are June 5 and June 6.

An investigation into the cause of the malfunction is underway.

"The leading suspect would be either a hardware problem or a problem with the network," ULA CEO Tory Bruno said at the Saturday press conference, noting that the investigation is not complete.

The launch's hold was automatically issued earlier Saturday for an unspecified reason, NASA said on its broadcast, with under four minutes remaining in the countdown. Holds in a rocket launch countdown — as well as "scrubs," indicating a launch delay — are a common occurrence in the industry.

The two NASA astronauts who were aboard the Starliner capsule, Butch Wilmore and Suni Williams, are safe.

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NASA and Boeing called off a launch attempt in early May due to an issue detected with the rocket. ULA, a joint venture of Boeing and Lockheed Martin, replaced the rocket's problematic valve.

After calling off the May attempt, NASA and Boeing found a "small" helium leak in Starliner, causing the agency and company to perform another series of assessments. After analysis, NASA and Boeing said they believe the source of the leak is in the spacecraft's helium propulsion system. But officials said May 24 that the leak is "stable" and "not a safety of flight issue."

Starliner's crew debut has been delayed for years. Boeing has eaten $1.5 billion in costs and nearly $5 billion of NASA development funds due to Starliner setbacks.

The spacecraft was once seen as a competitor to SpaceX's Dragon capsule, which has been flying astronauts for NASA regularly since 2020 under the agency's Commercial Crew program. However, various setbacks and delays have steadily slipped Starliner into a backup position for NASA, with the agency taking the rare step of reassigning astronauts off the first Boeing crew flights in 2021. Boeing is under contract to fly six operational Starliner missions to the ISS. The crew flight test that was scheduled for Saturday represents the final major step before it receives NASA certification to begin regular missions.

The astronauts

Astronauts Wilmore and Williams are still scheduled to fly on Starliner, with the former serving as the spacecraft's commander and the latter as its pilot.

Wilmore joined NASA in 2000 and has flown to space twice previously, on the space shuttle and Russia's Soyuz. Before joining NASA, Wilmore was a U.S. Navy pilot.

Williams was selected by NASA in 1998 and has also flown to space twice before, on the space shuttle and then the Soyuz. Williams also was a Navy pilot before joining the space agency.

The rocket and capsule

Starliner launches on ULA's Atlas V. The rocket debuted in 2002, and the Starliner crew flight test represents its 100th launch.

The capsule is built to carry as many as four NASA astronauts per flight and more than 200 pounds of research and cargo. The spacecraft lands using a parachute and airbag system. Starliner is reusable, and each capsule is designed to fly as many as 10 missions.

The mission

Boeing's crew flight test aims to certify the Starliner system as capable of carrying NASA astronauts to and from the ISS.

After Starliner launches, it will fly in space for about 25 hours before a planned docking with the International Space Station. The astronauts will then spend about a week on the ISS, focused on testing Starliner, before returning to Earth.

— CNBC's Rebecca Picciotto contributed to this report.