Balloonists from 10 countries have lifted off in one of the world's oldest aviation races.
To win the 20 teams have to try to travel the greatest distance from the launch site, near Bristol, in hydrogen-filled balloons.
Some of the competitors could be airborne for three or four days.
It is the first time in its 104-year history that the Coupe Aeronautique Gordon Bennett race has taken off from the UK.
Race launch director Clive Bailey said although the balloons were "very safe" there was always a risk.
"You potentially have a bomb above your head and then you have sea landings and landings at night with power cables so it's not a straightforward thing," he said.
"These are very brave guys."
Winds in the Bristol area are coming from the north and it is thought the balloons could travel as far as Italy or Portugal.
The balloons are controlled by releasing gas to go down and throwing out sand to go up.
A spokesman for the organisers said the wind was an issue and it caused a delay to the launch.
But the balloons had all taken off by 0130 BST.
The Duke of Edinburgh and Sir Richard Branson are patrons of the 54th Coupe Aeronautique Gordon Bennett race.
David Hempleman-Adams, from Wiltshire, won the race in 2008 with co-pilot Jon Mason, giving them the honour of hosting it in their home country.
Mr Hempleman-Adams and Mr Mason were the first UK team to win, flying 1,098 miles (1,767km) from Albuquerque in New Mexico to Lake Michigan in just over three days.
The pilots in command of the British teams for this years race are David Hempleman-Adams, Colin Butter and Janet Folkes.
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