Sky Bet suspend betting on Radamel Falcao's next club after seeing him backed at as short as 1/3 to join Real Madrid.
Morgan Schneiderlin has appealed to Southampton fans after a thwarted attempt to leave St Mary's.
This season Tello has made 16 appearances in all competitions for Barca, having represented Spain at this summer's Olympics.
A Barcelona statement read: "Barcelona announce they have reached an agreement with the player Cristian Tello to extend his contract until June 30, 2016. His buy-out clause has been set at 10 million euros.
"The player will sign his contract on Monday, December 17, in the Barcelona offices."