Cosmonauts worked round the clock

Earlier Russian ground controllers said the problems with the craft's main computer had been solved.

"The cosmonauts are now loading software into the station's computer," said a spokesperson from mission control in Moscow. "It underwent tests after being rebooted earlier and it looks like everything is working fine."

But a Nasa spokesman said the cosmonauts found that there was still a problem when they tried to reactivate other systems.

The computer broke down on Saturday, leaving the station adrift in orbit but in no immediate danger.

Although a replacement computer was installed on Sunday, Mir cosmonauts were unable to get it working properly because of a problem when downloading software.

The American shuttle cannot dock with Mir unless the station's automatic steering system is working and keeping the space station steady. Steering is controlled by the computer.

Thomas going home?

The countdown may be back on hold

The shuttle launch, which correspondents now say may be delayed, will take home Andrew Thomas - the last of seven American astronauts to serve on Mir.

The American mission will be the last before the space station is taken out of service at the end of June.