A British team attempting to break the world altitude record for a crewed balloon have called off their mission after a last-minute hitch.
At 0655 BST, with the envelope inflated to about 50 feet (15 metres), helium started to escape from the polythene structure and it collapsed.
The balloon had never been fully inflated before
The project has been put on hold while investigations are carried out. Early indications suggested a broken seam at the top of the QinetiQ 1 balloon might have caused the failure.
Pilots Colin Prescot and Andy Elson were attempting to beat a 42-year-old record set by two Americans who reached 113,740 feet (34,668 metres) over the Gulf of Mexico.
Mr Elson, who also designed the balloon, told BBC News all the crew were disappointed, but he added: "I'm totally convinced this is achievable.
HISTORIC BALLOON FLIGHTS
Pilots have constantly pushed the latest technology to fly their balloons ever higher and further
"This hasn't been done for 42 years and then it was done by a government. If it was easy, it would have been done in the meantime."
It is unlikely the project will get another attempt to launch this year.
"The balloon is so large that it is going to take at least two weeks to assess the whole thing - and the whole balloon will have to be rechecked - so we're probably going to miss this year's weather window," said Brian Jones, the mission control chief.
QINETIQ 1 - POSSIBLE EXPLANATION
(1) Twist occurs as balloon comes off storage drum
(2) Extreme pressure builds at top of main balloon
(3) Seam fails when envelope is just 50ft above deck
(4) Buoyancy envelopes suddenly fall back down
Technicians said it was possible a twist in the balloon as it came off its storage drum led to a pressure build-up at the top of the envelope splitting open a seam.
The ascent off the coast of Cornwall was already delayed 24 hours due to adverse cloud conditions on Tuesday.
But meteorologists' forecasts of a favourable north-easterly wind for Wednesday morning proved correct and the launch ship, which had been moored off St Ives, was moved to the lift-off zone about 10 to 20 miles (16 to 32 kilometres) out at sea.
The launch was due to take place at 0830 BST (0730 GMT).
At the time that the failure occurred, Andy Elson and Colin Prescot were being fitted into their spacesuits prior to climbing into the balloon's gondola.
"It was very sudden when the balloon came down," said Colin Prescot.
"It certainly wasn't something we expected. My immediate reaction was that the main valve must have come open by mistake - someone must have tripped it."
The balloon was meant to begin its ascent pencil-shaped and then become more rounded as it rose towards its target height - which it was due to reach within five hours of the launch.
For two hours, the pilots were to carry out scientific experiments and remotely fly a tethered solar-powered plane, the Zephyr 3, around the gondola to video their progress.
The QinetiQ 1 balloon is the biggest balloon ever constructed to carry people. Its skin is as thin as a freezer bag but its total size would match that of New York's Empire State Building.
It had been designed, modelled and built using computers. Although components and a scale model had been thoroughly tested, the final structure had never been fully inflated.
Brian Jones explained: "These extreme balloons are a one-shot endeavour. You simply can't take them out into the park and blow them up to see if they work.
Malcolm Ross and Vic Prather set the current record in 1961
"It's very disappointing but in some respects not that surprising. Remember, there were 22 attempts to fly around the world in a balloon before it was done successfully."
The altitude attempt was sponsored by QinetiQ, a science and development company spun off from the UK's Ministry of Defence.
One of the company's managing directors, Dr David Anderson, immediately said it was still committed to the project. He revealed QinetiQ had already invested £2m in the effort.