Russia announced a delay Monday in the planned launch of three astronauts to the International Space Station on March 30 due to a technical problem with the spacecraft.
The Soyuz launch from the Baikonur cosmodrome in Kazakhstan is of huge importance to Russia as it comes ahead of the 50th anniversary of the first human space flight by Yuri Gagarin on April 12.
The Soviet-era Soyuz system will provide the world's only link for human travel to the ISS after the US shuttle programme closes in the coming months.
Any problems with Soyuz spacecraft or rockets could pose a serious challenge to international space exploration.
The Russian Federal Space Agency said in a statement that a fault with one of the Soyuz capsule's systems had been uncovered in testing and that the launch would be delayed "until a later date".
Various news reports said the launch may now take place between April 7 and 10.
The delay was announced just two weeks after a top government official accused the space agency of committing "childish" errors that included the loss of three satellites in December.
RIA Novosti said the latest malfunction affected a switching system that allowed the crew to communicate with ground control.
"We have formed a task force involving the developers and producers as well as the parent organisation of the manned flight programme, RSC Energia," the space agency statement said.
Russia named the March 30 mission in honour of Gagarin, the pioneering cosmonaut whose historic space flight at the height of the Cold War is still feted as one of the country's most important achievements.
The mission will include the Russians Alexander Samokutyayev and Andrei Borisenko as well as NASA astronaut Ronald Garan.
Explore further: Image: Solar Electric Propulsion engine's ionizing Hall thruster