An attempt to set a new world altitude balloon record was cancelled today.
The decision was announced after the balloon began losing helium during inflation.
The setback occurred less than an hour after the mission had been given the go-ahead.
British balloonists Colin Prescot and Andy Elson had been set to launch the 1,270ft QinetiQ1 balloon between 8.30 and 9am.
But just before 7.30am the mission was cancelled.
Mission control director Brian Jones said said: “It lost a substantial amount of helium during inflation, probably due to a problem with the valve.
“Things were going incredibly well. Because of that I think it is an even more devastating blow for the team on board and us in mission control.”
He said the launch cancellation meant the team had missed the “weather window” and would have to put off any new attempt until next year.
The balloon, taller than the Empire State Building, was being inflated with helium aboard a vessel about 10-12 miles off St Ives, Cornwall.
Conditions for the launch, postponed from yesterday morning, had been described as “perfect” today.
Mr Jones said the exact nature of the problem was still not clear.
“It was decided to abort this attempt. We are not exactly sure what happened.”
He added: “We always said the most tricky part of the operation would be the initial stages of inflation and that proved to be the case.”
The balloonists were planning to set a new record altitude of 132,000ft, taking the balloon to the edge of space.
The balloon was being inflated on a launch platform on board the prototype trimaran warship Triton, which was sailing on pefectly calm glassy seas.
The pilots had already swallowed special temperature pills which measure their internal body temperature before their mission was aborted.
Pilots Andy Elson and Colin Prescot are to carry out initial investigations on board the launch ship to find out what went wrong before disembarking around lunchtime.
Mission control director Mr Jones said it could take the team up to 10 days to discover why the mission had failed.
He said tests were carried out on the giant nine-acre balloon in an aircraft hanger earlier this year but today was the first attempt at inflation.
Mr Jones said the balloon needed to be brought ashore for checking and testing, adding: “I cannot tell you what happened, we do not know. We are unsure whether there was a failure in the balloon or the inflation valve.
“We lost a very substantial amount of helium, which makes it impossible to carry out repairs on the ship and re-inflate.”
He said the project had been going “extraordinarily well” until this setback.
He said the balloon was intact and completely deflated on board the launch vessel Triton and added: “We will be able to find out what the problem was.”
He said it could have been a tear in the super-thin polythene balloon canopy or a problem with a valve.
He confirmed that the project had missed the weather window for a launch this year but said: “We are determined to keep the project going.
“It is one of those things.”
Mr Jones, who said such an altitude attempt had not been made since the Nasa record was set in the early 1960s, added: “All we have to do is to analyse what went wrong and make sure it does not happen again.
“There is no reason why we should throw this away now.”
David Anderson, managing director of project sponsor Quinetiq, said the mission team was “extremely disappointed” at what had happened.
“They have been living with this for a very long time. You can imagine how they feel now this has happened,” he said.