NASA is likely to further delay its program to send U.S. astronauts to the International Space Station using capsules developed by Boeing and SpaceX, according to a report published Wednesday by the Government Accountability Office.

"Further delays are likely as the Commercial Crew Program's schedule risk analysis shows that the certification milestone is likely to slip," the GAO said in the report.

"Additional delays could result in a gap in U.S. access to the space station as NASA has contracted for seats on the Russian Soyuz spacecraft only through November 2019," the report says. NASA "does not have a contingency plan for ensuring uninterrupted U.S. access."

Boeing and SpaceX did not immediately respond to CNBC requests for comment.

Commercial Crew is NASA's solution to once again launch U.S. astronauts from U.S. soil. Since the end of the Space Shuttle program in 2011, astronauts have flown aboard Russian Soyuz — at a cost to NASA of more than $70 million per seat. NASA's new program is competitive, with contracts up for grabs for Boeing to win with its Starliner capsules and SpaceX with its Dragon capsules.

The development programs for both companies' capsules have been steadily delayed, with each expecting to complete uncrewed test launches in August at the earliest. NASA was expected to certify Boeing in December 2019 and in January 2020, according to analysis earlier this year, but the GAO says further delays are expected. The existing timeline already causes a one-month gap, at minimum, in NASA's contracts for seats with Russia and the first launches of Boeing and SpaceX.

"Senior NASA officials told us that sustaining a U.S. presence on the ISS is essential to maintain and operate integral systems, without which the ISS cannot function," the GAO said.