NASA's Orion spacecraft, atop a United Launch Alliance Delta 4-Heavy rocket, sits on the launch pad before its first scheduled unmanned orbital test flight from the Cape Canaveral Air Force Station, Thursday, Dec. 4, 2014, in Cape Canaveral, Fla. (AP Photo/Chris O'Meara)

The first manned test flight of NASA's new rocket and space craft could be delayed by nearly two years, the agency said in a statement Wednesday. Previously, that flight was scheduled to take place in 2021, but now NASA said it could be pushed back to as late as April  2023, which it said "is consistent with funding levels in the president’s budget request."

The announcement was immediately criticized by Rep. Lamar Smith, the Republican chairman of the House Science, Space, and Technology Committee, who accused the Obama administration of "choosing to delay deep space exploration priorities."

"We must chart a compelling course for our nation’s space program so that we can continue to inspire future generations of scientists, engineers and explorers," said Smith (Texas).

The Space Launch System, NASA's new rocket, and the Orion capsule, manufactured by Lockheed Martin, are two of the agency's top priorities and the vehicles that it says will take humans into deep space, and eventually Mars. But the Government Accountability Office has said the agency’s human exploration program is plagued by “inconsistent and unrealistic schedule goals,” as well as “significant technical and funding issues.”

Its first unmanned test flight of SLS has already been delayed to 2018, and the GAO has said that even once astronauts are able to eventually fly on it "future mission destinations remain uncertain.”

Last year, NASA flew its new Orion capsule, without passengers, about 3,600 miles above the Earth. That was farther than any spacecraft designed for humans had gone since the Apollo 17 moon mission in 1972, a significant achievement that was cheered within the agency and on Capitol Hill.

Despite the delay, NASA Administrator Charles Bolden said in a statement that the agency's “work to send humans out into the solar system is progressing.” And he said it will eventually "enable humanity to set foot on the Red Planet, and we are committed to building the spacecraft and other elements necessary to make this a reality.”

Robert Lightfoot, NASA associate administrator, said the agency had to take steps to ensure "we are prepared for unforeseen future hurdles. We’re committing to this funding and readiness level to stay on the journey we’ve outlined to get to Mars.”