Reports have been suggesting an initial £8million transfer fee with a further £4million-worth of clauses. A percentage of the fee is understood to be going to the 25-year-old's two previous clubs, Brentford - from whom City bought Turner for £300,000 - and Charlton, where he started his career. The transfer is sure to have become the new club record, previously held by Sam Ricketts' sale to Bolton which was also for an undisclosed fee.
Despite a shaky start after being signed by Phil Parkinson in 2006 as Leon Cort's replacement, the centre back established himself in City's defence.
He went on to form a solid partnership with Wayne Brown in the promotion-winning season of 2007/08, when he was many fans' player of the year.
He was also the stand-out performer in City's debut Premier League season, and has until now played in every single minute of the club's top flight games. He has even captained the side recently in Ian Ashbee's and George Boateng's absences.
Phil Brown already said he has been in talks with a replacement centre half "just in case" Turner left, and has until tomorrow's 5pm deadline to complete a deal.
Paul McShane signed for the club yesterday in an unrelated deal with Sunderland. Having been here on loan last season, a permanent transfer has been agreed for around £2million. Although he can also play at centre back, he is expected to fill the gap at right back since the sale of Ricketts.