Richard Branson has insisted his Virgin Galactic enterprise will be ready for its first flight next spring.

In an interview with the Sunday Telegraph, the founder of Virgin Atlantic rejected claims by critics that the latest delay in lift-off for his spaceship was further evidence that the business would never make it off the ground.

Instead Branson told the newspaper that he would be on the first passenger flight of his new “spaceline” when it takes off next spring.

Virgin Galactic was founded in 2004, with initial plans of taking paying customers into space by 2007. In 2009, the company said there would be flights within two years and earlier this year, Branson predicted he would be on the space voyage by the end of the year.

Earlier this month, he reportedly acknowledged that his target had slipped again. However Branson told the Sunday Telegraph had now made a crucial breakthrough on rocket design, and that test flights would resume from the Mojave airbase in California.

He has now predicted that the craft will make its first test flight before Christmas.

“I’ve joked that this really is rocket science, and it’s proven much more difficult than we had ever expected,” he said.

“The fact is, the mother ship is working great, the spaceport in New Mexico looks great, but the thing that has held us up is the rockets for the spaceship. We have faced a lot of criticism for being late, but we cannot rush these things and we are building a business that is offering  return tickets.

“We have to feel completely confident about sending people into space. We’re going to take them up and bring them back.”

More than 700 celebrities, including Leonardo DiCaprio, Russell Brand and Katy Perry and Stephen Hawking have so far paid deposits of £154,000 for a two and a half hour voyage to just outside the atmosphere, around 60 miles above the Earth.