The 33-year-old Private, who lost his legs when his Land Rover was blown up in Afghanistan in July 2007, fell heavily as he came out through the famous black door following a meeting with the Prime Minister.

He was there, along with former England rugby captain Lawrence Dallaglio to publicise a St George's Day match between London Wasps and Bath at Twickenham in aid of Help for Heroes.

Fijian-born Derenalagi, a member of the 2nd Battalion The Mercian Regiment, laughed off the incident and soon recovered to pose for photographs with Brown and Dallaglio.

Derenalagi, who was in a coma for nine days after his accident, had recently returned from a training trip to the United States Olympic Committee Training Centre at Chula Vista, near San Diego, where he was one of six members of the British armed forces chosen to develop their sporting prowess.

During the trip he was identified as having the potential to compete at London 2012 in the shot put.

He said: "My heart is set on wearing a medal or two."