A Blues spokesman said they had been informed by a person close to Francis' family that he had been taken ill on Friday.
It is believed Francis was taken to Solihull Hospital and the club spokesman said they had been told that the former England striker had had a stent fitted and was making a good recovery.
Francis played for Blues between 1971 and 1979 before becoming England's first £1million player when he joined Nottingham Forest. He also managed Blues between 1996 and 2001.
A statement on the club's official website www.bcfc.com read: "Everybody at Birmingham City FC would like to send our best wishes to club legend Trevor Francis following news that he is recovering in hospital from a suspected heart attack.
"The former Blues striker and manager is much loved by everybody associated with the club and we'd like to wish Trevor a speedy recovery."