Space Exploration Technologies' Falcon 9 rocket is test fired on a refurbished oceanside launch pad at Cape Canaveral Air Force Station in Florida.

Washington - The first commercial spacecraft due to dock with the International Space Station will likely be delayed again as the company behind the rocket continues to test its computers.

“At this time, a May 7th launch appears unlikely,” spokeswoman Kirstin Brost Grantham said in a statement Wednesday. “SpaceX is continuing to work through the software assurance process with Nasa.”

Nasa said on its website that the launch was postponed but gave no deails about a future launch date.

The company had earlier postponed a planned April 30 launch to Monday allow the company time to test the systems that it will use to dock with the station.

Space X's Dragon spacecraft is to take a three-day flight to the ISS and undertake a series of complicated docking manoeuvres and tests to prove it can safely latch onto the orbiting station.

The unmanned Dragon capsule will blast off atop a Falcon 9 rocket from the Kennedy Space Centre in Florida and carry 521 kilograms of cargo, mainly food for astronauts living on the station. If all goes as planned it will return to Earth with 660kg of discarded cargo.

The Dragon is to remain at the station for several weeks for the ISS crew to unload cargo, before the robotic capsule re-enters the atmosphere and splashes down off the California coast.

The move is seen as a landmark in the development of a commercial spaceflight industry that is to carry astronauts aloft in coming years.

Nasa retired its ageing space shuttle fleet last year and plans to focus on developing craft to travel on longer missions, with their sights eventually set on Mars. It hopes to shift short-range flights to the ISS and elsewhere to the commercial space industry, but until then is reliant on Russian Soyuz craft to carry astronauts aloft. - Sapa-dpa