Johnson passed a medical and his signing beat the Friday noon deadline, enabling him to go into the squad for the derby match with Wolves.

Birmingham beat off competition from five or six other clubs for the services of Johnson, whose Blackburn contract was due to expire in the summer.

The 23-year-old was given permission to talk to other clubs by Blackburn boss Graeme Souness.

Blues boss Steve Bruce siad: "He is a player that I like and admire and we had been monitoring his situation.

"He is young - only 23 - and has all the right credentials that I am looking for," Bruce told Birmingham's official website.

"He has already played 100 times for Blackburn and has featured in the Premiership this season.

"We are a bit light in the midfield area and he will prove to be a good acquistion for us.

"He can play anywhere across the midfield and has wonderful energy.

"When we were alerted to Damien's availability we have jumped in there and made sure we got him.

"The lad has found himself behind the likes of Dunn, Duff, Flitcroft and Gillespie at Blackburn.

"He really wants to play first team football and I will give him that chance."