Bass, 23, had been given informal word that he was physically fit enough to take a seat on a Russian rocket flight to the International Space Station.

The news had come two weeks after he had a minor heart operation to rectify an irregular heartbeat.

But on Wednesday, Russian Aerospace Agency spokesman Konstantin Kreidenko said it had had no contact with the singer.

Mark Shuttleworth became the second space tourist in April

"We have received no requests from either him or his representatives, not to speak about signing any contracts," said Mr Kreidenko.

"Anyone has the right to undergo tests in the Institute of Biomedical Problems.

"But that doesn't mean that such person is considered to be a candidate for space flight."

Bass had expected to be officially certified fit at a ceremony on Wednesday after completing a series of physical and medical tests following the end of 'N Sync's latest tour.

He then planned to start five or six months of flight training at the cosmonaut centre in Russia's Star City on Monday and had scheduled a news conference in Moscow later this week to discuss his plans.

His adventure was also to be followed on a series of US television specials.

"He's in top physical condition, top mental condition," according to David Krieff, a Hollywood producer who had agreed to provide financial backing for Bass.

Space camp

Mr Krieff said his company, Destiny Productions, was arranging corporate sponsorship as well as the TV deal.

Bass would become the youngest person ever to go into space, Mr Krieff said.

He has held the ambition since childhood, when he went to a space camp.

If successful, he could follow in the footsteps of Dennis Tito and Mark Shuttleworth, the world's first space tourists, who reportedly paid $20m (£14m) each for their trips to the international space station.

But Mr Kreidenko said no decision had been made yet concerning the next space tourist, and refused to comment on possible candidates.

The Russian space agency has kept one seat available in the three-person Soyuz spacecraft set to launch in October.

Time is running short to fill the slot, however, because of the months of preparation the candidate would have to undergo.

A Russian cosmonaut will start training for the mission if it becomes clear that no space tourist will join the trip.