Boeing’s Starliner spacecraft, which was scheduled to take two NASA astronauts to the International Space Station on May 6, will remain on the ground for at least another week as engineers chase down a small helium leak.

The launch, which will aim to demonstrate a vehicle that would give NASA an additional option for carrying its astronauts to orbit, is now scheduled for June 1 at 12:25 p.m. Eastern from the Cape Canaveral Space Force Station in Florida. Additional opportunities to launch are available on June 2, June 5 and June 6.

The shift in timing adds weeks of delays to a project that is already years behind schedule. Officials from Boeing and NASA said they needed the time to understand the problem and develop workarounds.

On May 6, the astronauts Butch Wilmore and Suni Williams were already strapped in the Starliner when the launch was called off because of a malfunctioning valve in the second stage of the Atlas V rocket. That problem was unrelated to the Starliner capsule, but then engineers spotted a small helium leak in the Starliner’s propulsion system.

Over the next few days, the valve on the Atlas V was successfully replaced.

The helium leak turned out to be thornier. It was traced to a seal on a helium line leading to one of 28 small thrusters known as reaction control system engines. “Much like you would have on any piece of your plumbing at home, a faucet or anything like that,” Steve Stich, the manager of NASA’s commercial crew program, said during a telephone news conference on Friday. “There’s a seal that keeps that interface tight.”

Helium, an inert gas, is used to push propellants to the thrusters, and if too much helium is lost, the thrusters may not work properly.

Tests showed no leaks in the seals leading to the other 27 reaction control system engines, and engineers were confident that the single leak was manageable. There are no plans to replace the seal, which would require pulling Starliner off the Atlas V rocket and lead to an even lengthier delay for the flight.

“We could handle this particular leak if that leak rate were to grow even up to 100 times,” Mr. Stich said.

Mr. Stich said the helium leak led NASA and Boeing to take a wider look at the Starliner’s propulsion system, which revealed a “design vulnerability.” If a series of unlikely failures occurred, the spacecraft might not be able to bring the astronauts safely back to Earth.

If there were problems with the larger engines intended to be fired for the deorbit maneuver, one of the backup plans was to use eight of the smaller thrusters. However, the analysis showed that an additional failure might mean there would be only four of the smaller thrusters available.

The engineers then developed another backup plan to bring Starliner out of orbit with only the four thrusters.

If the valve problem had not occurred on May 6, Starliner would have launched, and the helium leak would not have been discovered until after it docked at the space station. But Mr. Stich said NASA and Boeing would have worked out a plan, and there probably would not have been any danger to the astronauts.

“We would have been able to manage it in flight just fine,” he said.