This article is more than 13 years old

This article is more than 13 years old

Three people have been killed in an explosion, during a test of rocket systems to be used in Richard Branson's proposed space tourism ventures.

Three other people were severely injured in what was described as a major blast at the Mojave Air and Space Port facility in California.

All those killed and injured worked for Scaled Composites, the private venture run by aerospace designer Burt Rutan, whose SpaceShipOne became the first private manned rocket to reach space.

The blast, shortly after 2:30pm yesterday (10.30pm UK time), happened during testing work on a rocket motor for SpaceShipTwo, which is being designed and built by Scaled for Mr Branson's proposed Virgin Galactic space tourism venture.

The company plans to offer commercial trips into space for tourists costing around £100,000 a head on SpaceShipTwo, which will carry six passengers and two pilots.

The president of Virgin Galactic, Will Whitehorn, expressed the company's sympathies to the families but would not comment further until Scaled had completed its investigation.

According to Mr Rutan, the explosion happened during tests of the flow of pressured nitrous oxide through an injector. The company uses nitrous oxide as an oxidiser in its rockets, providing the oxygen that rocket fuel needs to burn.

The cause of the blast was a mystery, a highly emotional Mr Rutan told a press conference at the former military airport.

"We were doing a test we believe was safe. We don't know why it exploded. We just don't know," he said.

Scaled uses part of the isolated airport, a former US air force base, as a testing facility for its space ventures.

A number of other companies - including other commercial space ventures - used the Mojave facility for testing, said Stuart Witt, the airport's general manager.

"What we do is inherently risky," he said. "These are not the days we look forward to, but we deal with it."

A local hospital spokesman said two of those killed died at the scene while the third died later in hospital. Two of the three injured workers were in a critical condition and one was serious. All were suffering numerous shrapnel wounds, the spokesman added.

In 2004, SpaceShipOne, funded by Microsoft co-founder Paul Allen, made the first privately financed manned spaceflight by climbing more than 62 miles on a suborbital journey above the Mojave desert. SpaceShipOne went on to make two more flights and won the £5m Ansari X Prize for the first private manned space flight.

Mr Rutan has since been developing SpaceShipTwo for Branson, who is investing at least £100m for a fleet of spaceships. The first vessel is planned to be ready within a year, and following a year of flight tests, have its first commercial launch in 2009.

US defence giant Northrop Grumman owns 40% of Scaled and has agreed to buy the rest of it, subject to regulatory approval.