The two cosmonauts and astronaut replaced the computer, which had failed just before the last US Discovery shuttle mission to the ageing Russian craft lifts off, mission control said.

A spokesman said that American Andrew Thomas and Russians Talgat Musabayev and Nikolai Budarin installed a spare computer, tested it and started downloading software.

The failure happened late on Saturday - for the second time since February - disrupting a system which keeps the solar panels of the space station oriented towards the Sun.

Systems shut down

It forced the crew to shut down systems to conserve electrical power on Mir.

Flight controllers told the crew to turn on the attached Soyuz capsule so its thrusters could be used to steer the station and realign the panels.

The crew also turned off their air conditioner, lights and other non-essential equipment to conserve power.

The countdown continues at Cape Canaveral

Despite the problem, the countdown began as planned for a Tuesday launch by Discovery.

The space station has faced a series of problems since it was involved in a collision in orbit almost a year ago.

Last year - the most dangerous in the station's 12-year history - computers continually shut down and caused power problems.

"We've seen this several times before, so it's something that the flight control team knows how to deal with," said Nasa spokesman Kyle Herring.

Similar computer failures in the past have taken just a day or two to fix - plenty of time before Discovery is due to take off, Herring said.

Russian space officials suspected the problem was with the computer's software and could be fixed quickly, possibly by Sunday night.

This will be the ninth and final time a shuttle links up with the ageing space station. The main objective is to pick up American astronaut Andrew Thomas, who has been living on Mir since January.