Two British pilots have delayed their attempt on the world altitude record for a manned balloon because of unfavourable weather conditions.
Colin Prescot and Andy Elson are attempting to better a 1961 mark set by two Americans of 113,740 feet (34,668 metres).
They were due to lift off on Tuesday from the southwest tip of England but concern about ice forming on their envelope has prompted the team to push back the effort by 24 hours.
"It was a difficult decision but a unanimous one." Prescot said. "We're a wee bit frustrated and disappointed."
Meteorologists had told them there was a strong likelihood that had they gone for lift-off on Tuesday morning, the balloon would have encountered cloud at 6,000, 9,000 and 16,000 ft (1,800, 2,700 and 4,900 m).
This could have resulted in ice forming on the balloon and preventing it from reaching a record breaking altitude. "We'd just heave our way up and heave our way down," Prescot said. "Wednesday looks much better."
Brian Jones, the team's mission control director, explained to reporters how ice could add a "substantial weight penalty" to an envelope.
Pioneers: Malcolm Ross and Vic Prather
He said that when he made the first circumnavigation of the globe in a balloon, his team had calculated that just one millimetre of ice could add six tonnes of mass. The QinetiQ 1 balloon was very much bigger, he said.
"We have a better chance of achieving our goal if we delay our flight by 24 hours," he added, confidently forecasting a 95% chance of getting airborne on Wednesday.
The balloon will take off from the stern of a prototype warship, RV Triton, stationed off St Ives in Cornwall.
If all goes well, the pair will get to their target height of 130,000 ft (39,500 m) within five hours of the launch. A splashdown should occur 50 to 100 miles (80 to 160 kilometres) out at sea.
BBC News Online will be streaming a live TV feed direct from the balloon throughout the flight.
The current record was set by US navy pilots Malcolm Ross and Vic Prather in Strato-Lab 5 which flew above the Gulf of Mexico.
Prather was killed in an accident when he was recovered from the water and the UK duo are well aware of the risks involved in their mission.
"There are a great many things that worry us but we've had two years to work on this and we've trained fully," Colin Prescot said.
"We've rehearsed all the potential problems and we believe we have back-ups for them all."
Andy Elson added: "We are trying to imagine what is going to trip us up and trying to make sure we don't have any embarrassing moments.
"The sooner we can get it all together and off the deck the safer we are. The most risky area certainly is the beginning."
Their QinetiQ 1 balloon is the biggest manned envelope ever constructed. When inflated, it will stand 1,270 ft (390 m) tall - about the same height as New York's Empire State Building, or seven times the height of Nelson's Column in London.
HISTORIC BALLOON FLIGHTS
Pilots have constantly pushed the latest technology to fly their balloons ever higher and further
The polyethylene envelope - which has the same thickness as a freezer bag - is designed to carry 44 million cubic ft (1.25 million cubic m) of helium.
It should be visible to people on the ground for hundreds of kilometres around.
The pilots - hooked up to a supply of pure oxygen and wearing pressure-resistant spacesuits designed by Russia's Zvezda company - will ride in an open gondola.
Scientists say the pair will face temperatures as low as -70C and high levels of ultraviolet radiation as they rise to almost four times the altitude of a commercial airline flight.
Brian Jones: Ice is a "substantial weight penalty"
The duo will remain seated all through the mission but they will have only limited time to enjoy the view.
There are cosmic radiation and micrometeoroid experiments to be performed and Mr Prescot will try to fly a solar-powered propeller-driven plane, Zephyr 3. Only rocket planes and the shuttle have gone higher than the target altitude expected of Zephyr.
The plane will be tethered to the flight deck and will video the progress of the record attempt.
"We need to break the existing record by a minimum of 3% but perhaps the most difficult thing is that the pilots have to come back down to Earth and land in the water with the balloon," said Brian Jones.
"They can't simply cut the balloon away and parachute down, which was the case several years ago, and that's a really tricky thing to achieve."
Any record claim made by Mr Prescot and Mr Elson will be scrutinised by the Federation Aeronautique Internationale (FAI), the world's air sports federation.
Mr Elson, 48, from Somerset, is a balloon designer and engineer. Mr Prescot, 53, from Hampshire, is a commercial hot air balloonist and aerial filmmaker.
They already hold a number of ballooning records including the world endurance record for any aircraft in the Earth's atmosphere - a journey that lasted 17 days, 18 hours and 25 minutes.
"The history of this project is amazing," Prescot said. "In the 50s and early 60s, the Russians and the Americans were sending men up in balloons to find out how to survive outside the atmosphere.
"When Gagarin went into space on top of a 'roman candle' everyone forgot about the balloonists but they were the pioneers."
The altitude attempt has been sponsored by the QinetiQ company, a science and development spin-off from the UK's Ministry of Defence.