British billionaire Richard Branson has soared more than 80 kilometres above the New Mexico desert aboard his Virgin Galactic rocket plane and safely returned in the vehicle's first fully crewed test flight to space, a symbolic milestone for a venture he started 17 years ago.

A massive carrier plane made a horizontal take-off from Space Port, New Mexico at around 8.40am Mountain Time (3.40pm Irish time) and ascended to an altitutude of 15 kilometres.

The mothership then dropped a rocket-powered spaceplane called VSS Unity, which ascended at Mach 3 beyond the 80 kilometre mark carrying two pilots and four passengers, including Mr Branson.

Once the rocket engine is cut off, passengers can unbuckle and experience a few minutes of weightlessness, while admiring the curvature of Earth from the ship's 17 windows.

After peaking at around 88km of altitude, the ship re-entered the thick part of the atmosphere and glided back to the runway.

"It's a beautiful day to go to space," Mr Branson wrote in a tweet earlier where he posted a video of himself biking to the base and meeting with his crewmates, all Virgin employees.

Mr Branson was also aiming to one up Amazon founder Jeff Bezos by winning the race to be the first tycoon to cross the final frontier in a ship built by a company he founded.

Earlier, he posted a picture of himself standing in a kitchen with SpaceX boss Elon Musk, who'd come to show his support.

Several tourists journeyed to the International Space Station in the 2000s, but on Russian rockets.

Mr Branson's official role is to evaluate the private astronaut experience to enhance the journey for future clients.

Mr Branson, who founded the Virgin Group that now has interests in everything from commercial aviation to fitness centers, is known for his appetite for adventure and has set world records in hot air ballooning and boating.

"As a child, I wanted to go to space," the 70-year-old wrote a few days ahead of his trip.

He founded Virgin Galactic in 2004, but the dream almost came to an end in 2014 when an in-flight accident caused the death of a pilot, considerably delaying the program.

Since then, VSS Unity has successfully reached space three times, in 2018, 2019 - which included the first crew member who wasn't a pilot - and finally May this year.

This latest spaceflight took off from Spaceport America, a huge base built in the Jornada del Muerto desert, around 30km southeast of the nearest dwelling, Truth or Consequences.

Financed largely by the state of New Mexico, Virgin Galactic is the principal tenant.

Virgin Galactic plans two further flights, then the start of regular commercial operations from early 2022. The ultimate goal is to conduct 400 flights per year.

Some 600 tickets have already been sold to people from 60 different countries - including Hollywood celebrities - for prices ranging from $200,000 to $250,000.

And though, according to Mr Branson, "space belongs to us all," the opportunity for now remains the preserve of the privileged.

"When we return, I will announce something very exciting to give more people the chance to become an astronaut," he promised.

The competition in the space tourism sector, whose imminent rise has been announced for years, has come to a head this month.

Mr Bezos, the richest man in the world, is due to fly on 20 July on Blue Origin's New Shepard rocket.

Blue Origin posted an infographic Friday boasting the ways in which the experience it offers is superior.

The principal point: New Shepard climbs up to more than 60 miles in altitude, thus exceeding what is called the Karman line, the frontier of space according to the international convention.

"None of our astronauts have an asterisk next to their name," the company bragged.