JOIN USThere was no immediate word on the condition of the pilot.The Ministry of Defence said it was aware of an incident involving the Red Arrows and was investigating. It gave no further details.
The BBC and Sky News said the Royal Air Force jet came down near Bournemouth, where an air show is being held today.
Witnesses said nine planes took off from Bournemouth Airport to perform a display, but only eight landed. Local resident Shaun Spencer-Perkins said the plane came down in fields, near a river.
"I heard a rushing sound and I saw a plane about 15 meters (50 feet) above the ground racing across the fields," he told the BBC. "It impacted and bounced across the field."
He said two members of the public jumped into the river to search for the pilot. The Bournemouth Air Festival said events were continuing as scheduled.
The Red Arrows are famous for their airborne stunts, multicoloured vapour trails, dramatic flypasts and trademark diamond formation.
Formed in 1965, the Arrows have flown more than 4,000 displays in 53 countries. Their red single-engine jet trainers are a familiar sight at air shows and military events.
The nine-pilot stunt team last had an accident in March 2010, when two jets crashed in training in Crete. Neither pilot was seriously injured in that incident.