The 24-year-old's contract at Middlesbrough had expired and he has opted to stay in the Barclays Premier League rather than play in the Championship with Middlesbrough.

Turnbull is a former England youth international who made his first team debut in May 2006 against Bolton Wanderers.

He made 29 senior appearances for Boro, and 67 out on loan at Darlington, Bradford City, Barnsley and Crewe Alexandra.

He is likely to be battling Hilario for the number two goalkeeper's jersey, with Petr Cech firmly established as first-choice.

Meanwhile, the 33-year-old Hilario signed a new deal with the Blues on Thursday, tying him to the club until 2011.