ONE of the biggest let-downs in the history of ballooning yesterday scuppered hopes of a record-breaking trip from the coast of Cornwall to the edge of space.

The attempt by two UK balloonists to set a new world altitude record was called off barely an hour before launch after their balloon developed a split as it was being inflated.

The decision was made after the 1270ft QinetiQ1 began losing helium aboard the launch ship, about 20 miles off the west coast of Cornwall. It now will be next year before the return of a suitable weather window for the venture.

Disappointed pilots Colin Prescot and Andy Elson were due to launch between 8.30am and 9am yesterday. However, the mission was dramatically cancelled just before 7.30am, three-and-a-half hours after the inflation process started.

The launch had been postponed from Tuesday morning because of cloudy weather, and conditions yesterday were perfect. But the balloon - made of polyethylene as thick as a freezer bag - twisted as it unrolled from its drum during inflation. The QinetiQ1 was to have been the world's biggest manned helium balloon, taller at 1270ft than the Empire State Building, but a seam split when the helium in the top of the envelope came under pressure.

The damage was beyond the scope of the rolls of sticky tape on board the launch ship for emergency repairs.

Brian Jones, mission control director, said the launch had been aborted after the balloon lost a substantial amount of helium.

''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.

''We now need to do a thorough inspection. The reason for the abort is we did not have enough helium to repair and refill, and this was always a one-off shot with the balloon.

''There is no additional envelope, and there is only one drum that fits on the ship.''

At sea level, the balloon required just 4000 to 6000 cubic metres of helium to fully inflate its envelope. By the time it had reached an altitude of 132,000ft, the helium would have expanded 360 times, filling a space of 1.2 million cubic metres.

The project now had missed the ''weather window'' for this year, Mr Jones added.

The balloonists were planning to set a new record altitude of 132,000ft, taking the balloon to the edge of space.

The balloon, sponsored by QinetiQ, the UK science and technology research company, was expected to have been visible from as far away as London during its flight.

When the mission was called off, the pilots, Mr Prescot, 53, from Stockbridge, Hampshire, and Mr Elson, 50, from Wells, Somerset, were about to climb into their pressurised space suits. They had swallowed special temperature pills which measure their internal body temperature.

Their target was the 113,740ft record set in 1961 by two US Navy pilots as part of the US space programme.

If the flight had gone ahead, they would have spent eight hours strapped to an open gondola before splashing down in the Atlantic.

The QinetiQ record attempt, more than two years in the planning, was postponed from last year because of unsuitable weather, and this year the team had waited since July for a suitable weather window. However, Mr Jones said: ''We are determined to keep the project going.''