If successful, it would be a major step toward the long-delayed dream of commercial space tourism.

The next test flight could come as early as Thursday with two pilots taking Virgin Space Ship Unity high above California's Mojave Desert.

A company statement says the next stage of testing aims to "reach a space altitude for the first time".

"Although this could happen as soon as Thursday morning, the nature of flight test means that it may take us a little longer to get to that milestone," the statement said.

Space begins at an altitude of 100 kilometres; the last test flight was at 52km.

Reaching that space threshold would demonstrate significant progress toward the start of commercial flights that were promised more than a decade ago.

Virgin Galactic's development of its spaceship took far longer than expected and endured a setback when the first experimental craft broke apart during a 2014 test flight, killing the co-pilot.

More than 600 people have committed up to $US250,000 ($A346,000) for rides in the six-passenger rocket, which is about the size of an executive jet.

Branson isn't alone in the space tourism business: Jeff Bezos' Blue Origin is planning to take space tourists on suborbital trips, using the more traditional method of a capsule atop a rocket that blasts off from a launch pad.

SpaceX's Elon Musk recently announced plans to take a wealthy Japanese entrepreneur and his friends on a trip around the moon.

Australian Associated Press