The England Under-21 central defender will undergo the examination with the Barclays Premier League newcomers before sealing his move to St Andrews.

Villa boss Martin O'Neill confirmed: "It should all go to plan if he passes a medical."

Blues boss Steve Bruce has been trying to sign Ridgewell since the start of the year.

Birmingham had a $5million bid rejected in January and also failed with an attempt to take Ridgewell on loan before the transfer window closed after the departure of Matthew Upson to West Ham.

Bruce has continued to monitor Ridgewell's situation and a fee of around $6.25million is finally set to persuade Villa manager Martin O'Neill to part with the player.

Ridgewell, for whom agreeing personal terms will not be a problem, will be the first player in recent times to make the direct switch between the two fierce second city rivals.

But it has been commonplace in the not too distant past with the likes of Tony Morley, Robert Hopkins, Dennis Mortimer and Alan Curbishley playing for both clubs.

Ridgewell, a former West Ham trainee, was a regular first-team player under David O'Leary during the 2005-06 campaign and also featured for most of the first half of O'Neill's first season at the helm.

But he has slowly slipped down the pecking order and now finds Olof Mellberg, Martin Laursen and Gary Cahill ahead of him.

The need to have a greater chance of playing first-team football has persuaded Ridgewell to make the move to St Andrews and has over-ridden any fears he may have in terms of the treatment he may receive from Birmingham fans.

Blues would make no comment on the proposed deal today but an official announcement is expected sooner rather than later.

Bruce has stressed the need to strengthen at the back 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."