The next attempt will be May 21 at the earliest.
The high-stakes mission was already called off last week, with two astronauts strapped in and ready to fly.
That was because engineers detected a problem with a valve that regulates liquid oxygen pressure on the Atlas V rocket meant to propel Starliner into orbit.
The valve was replaced and a new launch date of May 17 was set.
But a new problem emerged -- a small helium leak in a part of Starliner called the service module, which holds instruments used to control and operate the spacecraft.
Engineers need to carry out tests as they try to fix the problem, Boeing said in a statement.
"Helium is used in spacecraft thruster systems to allow the thrusters to fire and is not combustible or toxic," Boeing said.
Liftoff from Florida is now scheduled for 4:43 pm (2043 GMT) on May 21.
Astronauts Butch Wilmore and Suni Williams have returned to Houston as teams work to fix this new glitch and will return to Florida over the next few days, Boeing said.
The mission has already faced years of delays and comes at a challenging time for Boeing, as a safety crisis engulfs the century-old aerospace titan's commercial aviation arm.
NASA is banking on Starliner's success in order to achieve its goal of certifying a second commercial vehicle to carry crews to the International Space Station.
Elon Musk's SpaceX achieved the feat with its Dragon capsule in 2020, ending a nearly decade-long dependence on Russian rockets following the end of the Space Shuttle program.
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