Nasa is pressing ahead with Monday's planned launch of a supply ship despite a critical computer outage at the International Space Station, having determined that the situation is safe.

Mission managers decided on Sunday to proceed with the countdown for the SpaceX Dragon capsule, which is already a month late in delivering more than two tons of cargo. "We're good to go," said Nasa space station program manager Mike Suffredini.

Suffredini noted the many important supplies aboard the Dragon, including a new spacesuit and repair parts for the older spacesuits already in orbit. "We need to get it on board as soon as we practically can," he told reporters.

The backup computer, located on the outside of the space station, stopped working on Friday. The main computer kept operating perfectly, and the six-man crew was never in any danger. Nasa debated whether to delay the SpaceX mission and, on Sunday, determined the station has sufficient redundancy to safely support the visiting vessel.

A spacewalk will be required to replace the bad computer. Engineers don't know why it failed.

Suffredini said the spacewalk will be conducted by a pair of astronauts on 22 April, using suits outfitted with new fan components to avoid the near-disaster that occurred last summer. An Italian astronaut almost drowned when his helmet flooded with water from the suit's cooling system.

A 22 April spacewalk will give SpaceX two chances to get its unmanned Dragon capsule flying. Good weather is forecast for Monday's 4.58pm launch. If that doesn't work, the next launch attempt for the California company's Falcon rocket would come on Friday.

Nasa is paying SpaceX Space Exploration Technologies and Virginia-based Orbital Sciences to make station deliveries.

As soon as the Dragon soars, the space station's solar panels will be moved into the proper position for its arrival, Suffredini said. That will guard against any complications resulting from additional computer breakdowns.

More than a dozen of these computers, called MDMs or multiplexer-demultiplexers, are located on the exterior of the space station.