The mothership which will launch the first space tourists into the atmosphere was unveiled by Sir Richard Branson in California yesterday.

WhiteKnightTwo (WK2), the carrier aircraft which will be used to launch SpaceShipTwo into orbit, "will allow thousands of people to realise their dreams", Sir Richard said.

Speaking at the launch ceremony in the spaceship's hangar at the Mojave Air and Spaceport in California, the British billionaire, who will be among the first space tourists with his family, said the venture would help the world "wake up" to the fragility of the planet and the importance of protecting Earth.

"The roll-out of WhiteKnightTwo takes the Virgin Galactic vision to the next level and continues to provide tangible evidence that this most ambitious of projects is not only for real but is making tremendous progress towards our goal of safe commercial operation," he said.

Sir Richard said the first tourists could be launched into space in around 18 months' time. More than 250 customers have paid £100,000 for the chance to be one of Virgin Galactic's first space tourists.