MR Richard Branson and his crew aboard the Virgin Challenger balloon were being forced to land within the next six hours due to a technical problem, a Virgin spokeswoman told PA News early this morning.
The balloon was expected to land in North Africa and all on board were said to be well.
"We can't tell you any more, at this moment," said the spokeswoman.
Earlier the crew had been reported "on a high" as darkness fell at the end of the first day of its round-the-world mission.
Mr Branson's spokesman, Mr Will Whitehorn, who was flying in a chase plane behind the balloon until dusk, said the craft was near the Algerian border at 30,000 ft, travelling at about 28 knots (32 m.p.h.), heading slowly towards the Jet Stream.
"The crew are in very good spirits - and the balloon looks absolutely spectacular," he said.
"When it gets to 30,000 ft it is nearly three times the size that it is on the ground, because the helium expands so much with it."
The Virgin Challenger took off from Morocco at 11.18 a.m. (Irish time) yesterday with Mr Branson, Mr Per Lindstrand and a substitute co-pilot, Mr Alex Ritchie. Mr Ritchie replacd Cork born Mr Rory McCarthy who was struck by a sudden illness.
Mr Whitehorn said the crew was continuing to test all the systems on board - all of which seemed to be working effectively.
"They are very, very excited. You can tell they're on a real high. One of their biggest problems now will be trying to get to sleep."
Last night, the three pilots dined on a meal of packet astronaut-style food, heated in the capsule's microwave. They also, drank a large quantity of water.
Because they are travelling in a pressurised space-style capsule, the pilots need to drink about 10 or 11 pints of water a day to avoid dehydration.
"It's the largest balloon ever built and no one has ever flown anything like this before," said Mr Whitehorn.