The rocket NASA plans to use to get astronauts to the moon by 2024 has for years suffered significant cost overruns and schedule delays. But those problems are even worse than originally thought, according to a federal watchdog report expected to be released Wednesday.

The report, a copy of which was obtained by The Washington Post, said NASA had masked the true price tag of the program by shifting some costs to future missions without accounting for them. It accused the space agency of “a lack of transparency . . . especially for its human spaceflight program" and said NASA misrepresentations made it hard to determine the true cost of the program.

It said the cost of the rocket, known as the Space Launch System, had grown by nearly 30 percent or nearly $2 billion and that the first launch of the rocket, initially expected in late 2017, might not happen until June 2021.

Still, NASA has continued to pay tens of millions of dollars in “award fees” to Boeing, the SLS’s primary contractor, for scoring high on performance evaluations.

After issuing one award fee to Boeing, a NASA official even “noted that the significant schedule delays on this contract have caused NASA to restructure the flight manifest for SLS,” the report said. The GAO called for NASA to use “ongoing contract renegotiations” to “reevaluate its strategy to incentivize contractors to obtain better outcomes.”

The report comes as NASA is seeking congressional support for its plan to return humans to the moon within five years. Originally, NASA had planned to do that by 2028, but the Trump administration requested that the timetable be accelerated. To meet that demand, NASA recently requested an additional $1.6 billion from Congress for its moon effort, dubbed Artemis. Last week, NASA Administrator Jim Bridenstine told CNN that the cost of the program would be far more: $20 to $30 billion over five years.

But the key to that mission is the SLS, a rocket with a cost that is not fully known since “NASA’s current approach for reporting cost growth misrepresents the cost performance of the program,” according to the GAO.

Last year, NASA’s Inspector General blamed Boeing for a 2.5-year delay for the program.

In its latest report, the GAO said that NASA diverted nearly $800 million of program costs to future missions, which it said “obscures cost growth” for the program. It also noted that NASA cost estimate runs only through the first launch of the rocket, meaning there is a lack of transparency into what the total cost of the program is.

The GAO report also took aim at the program to develop the Orion spacecraft that would fly atop the SLS. The prime contractor for that program is Lockheed Martin, which also has been receiving regular award fees from NASA, despite being over budget.

A NASA spokesperson declined to comment Tuesday. But in a letter attached to the report released Wednesday morning, William Gerstenmaier, NASA’s associate administrator for human spaceflight and operations, strongly defended the agency’s management of the program, saying it has been transparent about the costs. He accused the GAO of repeatedly projecting “the worst-case schedule outcome” and said NASA takes “exception to the unnecessarily negative language” in the report.

“The GAO report does not acknowledge NASA is constructing some of the most sophisticated hardware ever built,” he wrote. “Like all other development programs, the challenges we face are significant but not insurmountable.”

Boeing said in a statement that the SLS “is critical to the nation and to the future of human spaceflight,” and that it was continuing to make progress. The core stage integration, assembly and testing of the rocket to be used in the first mission are in “the final stages," it said. The company is also building the core stage for the second Artemis flight. And it said it recently “restructured our leadership team to better align with current program challenges” and to help it “transition from rocket development to rocket production.”

Cost isn’t the only problem. The GAO report noted that SLS’s launch date has been a moving target. It was initially expected to launch in 2017, then late 2018, then December of 2019 and then June 2020. But even that date “is unlikely,” the GAO said, noting that “the first launch may occur as late as June 2021 if all risks are realized.”

It said that NASA and Boeing “underestimated the complexity of manufacturing and assembling the core engine stage section” where the engines mate to the rocket. There was a problem of debris in the fuel lines. In order to “keep costs low” Boeing initially hired only 100 technicians, but now has about 250 on the program.

Another issue was that Boeing began work on the rocket before it had a set of “detailed directions on how the vehicle should be built.” This slowed progress since “technicians can only perform work that they have instructions to carry out.”

If the SLS is significantly delayed, that would be a huge problem for NASA’s ability to get to the moon quickly. The agency has considered cutting out a major engine test known as the “green run” in order to speed up development. And Bridenstine even briefly considered using another rocket instead of the SLS for the first launch, which is planned to take an uncrewed spacecraft in orbit around the moon on a three-week mission. That would precede missions with astronauts who would eventually land on the lunar surface.

Democrats in Congress, meanwhile, have expressed skepticism about the Artemis plan, and it’s unclear whether NASA will get the money it says it need to pull off a lunar landing on such on an expedited timeline.

Bridenstine has repeatedly said that NASA would not cut other programs within the agency to fund Artemis. But Gerstenmaier, the NASA associate administrator, said that eventually it might have to do just that.

“We’re going to have to look for some efficiencies and make some cuts internal to the agency, and that’s where it’s going to be hard,” he said, according to SpaceNews.