(CNN)A government watchdog agency is warning NASA that its astronauts could be grounded by delays facing SpaceX and Boeing.
The Government Accountability Office on Wednesday released its latest report on NASA's commercial crew program, which has SpaceX and Boeing racing to develop crew-worthy spacecrafts that can carry astronauts to and from the International Space Station.
In its strongest warning yet, the GAO said NASA needs to have a contingency plan in place should delays faced by both companies leave NASA without any options for getting people to the space station.
"Without a viable contingency option [...] we found that NASA was at risk of not being able to maximize the return on its multibillion-dollar investment in the space station," the GAO report says.
The United States has not had the ability to fly humans into orbit since the space shuttle program ended in 2011. Since then, NASA has paid for seats aboard Russian spacecrafts.
But those seats are only guaranteed through November 2019. And the GAO estimates Boeing's Starliner spacecraft and SpaceX's Crew Dragon will not be certified for their first missions until December 2019 and January 2020, respectively.
That schedule leaves little wiggle room for any additional delays, according to the GAO, and securing additional seats on a Russian Soyuz spacecraft could take years.
NASA said it is "continuing to assess multiple scenarios to ensure continued" access to the space station.
One option NASA has floated: Extending the length of the first crewed test mission of Boeing's Starliner from about two weeks to six months. That way, the test could serve more as a full mission. NASA said in April that it adjusted its contract with Boeing to open that up as a possibility.
NASA has an ambitious timeline for the commercial crew program on its official web page. It states SpaceX will conduct its first crewed test flight by December of this year, and Boeing will fly astronauts in November.
But the GAO report says NASA officials said last month those targets will likely change.
NASA awarded Boeing and SpaceX fixed-priced contracts — worth $4.2 billion and $2.6 billion — in 2014. And, like the vast majority of spacecraft development programs, they've been bogged down with delays since. NASA originally projected the commercial crew program would be up and running in 2017.
The commercial crew program is modeled after NASA's commercial resupply program, which tapped SpaceX and Orbital ATK to make uncrewed resupply trips to the space station on behalf of NASA. That program was novel because it allowed companies to run the show by designing their own rockets and spacecrafts, rather than putting that burden on NASA.
NASA hopes to piggyback off the success of the resupply program with commercial crew, which has Boeing and SpaceX racing to become the first in the private sector to send a human into orbit.