James DeGale and Nathan Cleverly have been sparring this week ahead of their May 21 fights at the O2 Arena.
Cleverly has traveled to Essex to go dozens of rounds with British super-middleweight champion DeGale.
The sparring is rumoured to have been full on, with both boxers looking sharp.
Cleverly takes on Germany's Jurgen Braehmer for the WBO light-heavyweight title, while DeGale defends his British title against Commonwealth champ George Groves.
"Sparring this week certainly wasn't for the faint hearted," said one boxing insider.
"Neither guy held back, and they traded some cracking shots.
"It's funny, they were really going at it in the ring, but as soon as sparring stopped they were laughing and joking like the friends they are. That was really good to see."