Robbie Williams and Gary Barlow will perform together on stage today for the first time since Williams quit Take That.
The pair will perform to 60,000 concertgoers at Twickenham Stadium in south west London in aid of the Help For Heroes charity which supports members of the armed forces.
Their set will include the debut live performance of their new single Shame.
Earlier this year Williams ended months of speculation by announcing he was reuniting with his former bandmates for a new album.
Williams, who married his girlfriend Ayda Field in California last month, said: "I am honoured to be a part of this show and to help draw attention to the amazing job done by our servicemen and women throughout the world. Their dedication to our country is phenomenal and we are all so proud of them."
The concert will also feature performances by James Blunt, Alexandra Burke, Pixie Lott, Plan B and The Saturdays.
Comedians including John Bishop, Jack Dee and Peter Kay also signed up to host the event which features a guest appearance by Bruce Forsyth.
Free tickets were given to injured members of the armed forces and their carers.
Bryn Parry, of Help for Heroes, said the charity allowed ordinary people to support the armed forces.
He said: "Music and humour can reach and lift those in the darkest of places, making life worth living and helping in recovery. Our boys and girls, both those operating in the remote patrol bases and those recovering from life changing injuries, will hear this concert and know that they are in the forefront of our minds. They are not forgotten, wherever they are."