Birmingham have signed Fulham defender Franck Queudrue in a deal that could eventually be worth £2.5m.
Queudrue, 28, has signed a three-year contract at St Andrews.
Birmingham boss Steve Bruce will now move to make it a defensive double by completing the signing of Aston Villa's Liam Ridgewell, also for £2.5m.
The 23-year-old is undergoing a medical on Friday and will become the first player in the last decade to make the switch from Villa to Birmingham.
Blues had a £2m bid for Ridgewell rejected in January and also failed with an attempt to take him on loan before the transfer window closed.
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The need to have a greater chance of playing first-team football has persuaded Ridgewell to make the move to St Andrews.
And that has left Villa boss Martin O'Neill viewing Norwegian international Brede Hangeland as a possible replacement for Ridgewell.
The 26-year-old defender has also been linked with a move to Newcastle and is likely to cost £4m.
Birmingham boss Steve Bruce has stressed his desire to strengthen his defence, particularly with left-back Mat Sadler and central defender Rafael Schmitz injured.
He said: "At the moment my priority has to be at the defensive end of the team.
"Mat Sadler has got a knock and we haven't really got anybody to replace him.
"In the centre-back area we've only got two at the moment - apart from Rafael Schmitz, who is also injured.
"That's not very healthy either and we are looking at those two positions."
Quedrue's arrival gives Bruce competition for places in the left-back berth, where his only choice at present is Matt Sadler, who is currently injured.
The Frenchman has a host of Premier League experience to call upon following his move to the Riverside Stadium from Lens five years ago.
He spent four seasons on Teesside, helping Boro win the League Cup in 2004 as well as the final of the Uefa Cup two years later before moving to Fulham last summer.
Newcastle have also been linked with Queudrue but Bruce has won the race for his signature.
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