The 70-year-old former Tottenham boss was brought in on a rescue mission at the end of last season and succeeded in keeping Blues in the Championship.
Despite signing 14 players during the summer transfer window, Birmingham have managed just one win from six games this time around.
Former Everton midfielder Lee Carsley, part of Redknapp's backroom staff, has taken over as caretaker manager and is tipped as the early favourite to keep the job on a permanent basis, with former Leicester City boss Nigel Pearson and ex-Middlesbrough coach Aitor Karanka also tipped as possible replacements.
Redknapp initially worked without asking for payment from Blues for the final three games of 2016-17, having replaced Gianfranco Zola.
Asked if this job would be his last he said'Every chance... I doubt it'll happen now very much
And after today's 3-1 home defeat to Preston, Birmingham made a decision to sack the 70-year-old.
Yesterday's defeat left Birmingham second from bottom in the Championship, with one win from eight matches.
"We have got to lift ourselves".
Port Vale's CEO, Colin Garlick, has revealed that David Kelly and Chris Morgan have been placed in charge of the first team while the club's search for a new manager, with both men joining the likes of Justin Edinburgh, Neil Redfearn and Neil Aspin in the betting to replace Brown.
"I am disappointed. It's been hard, I have had one week with a new team and new players, one week", he told the Birmingham Mail.