The 27 year old never really made the grade at Old Trafford and has slipped down the pecking order since he had his chances in the first team. Foster has made just 23 appearances for United in that time and spent a season on loan with Watford.
Birmingham’s on-loan keeper Joe Hart is now back at Eastlands and his career has come on leaps and bounds since joining the second city club.
Foster said: "You see what it has done for Joe Hart this season. He's shown everyone what he can do at Birmingham and now he's going to the World Cup.
"You never want to leave Manchester United. If you leave this club you're only ever going to go down. But I want to be playing football. I'm not interested in just hanging about."