Virgin Galactic was forced to abort a key test flight of its space plane after an apparent issue with its rocket motor.

Sir Richard Branson's space tourism company had launched the plane up to a height of 40,000 feet over New Mexico when it developed an issue and had to return to earth.

The test flight had aimed to reach a height of 50 miles above the planet to test the cabin experience and the boosters.

The firm confirmed that the space plane, which was carrying test pilots CJ Sturckow and Dave Mackay, safely landed back at its base.

"The ignition sequence for the rocket motor did not complete," the company said on Twitter.

 "Vehicle and crew are in great shape. We have several motors ready at Spaceport America. We will check the vehicle and be back to flight soon."