Birmingham have accused Aston Villa of "tapping up" Alex McLeish and intend to make a formal complaint against their local rivals.
Blues are upset at Villa's statement in which they indicated they are to press ahead with plans to interview McLeish for their own managerial vacancy.
The Scot resigned as manager of City on Sunday via an email to Birmingham acting chairman Peter Pannu.
Birmingham want £5.4million in compensation from Villa or any club who want to make McLeish their manager, to cover the remaining two years of his contract.
Villa's claim that they regard McLeish, nicknamed Eck, as a free agent because he resigned has incensed Blues.
City, in a media statement, claim: "Birmingham City find Aston Villa's announcement today (Tuesday) to proceed with an interview with Alex McLeish a contempt of Premier League and FA rules.
"The club feel this will taint football and give the game and the footballing authorities a bad name if this is allowed to materialise.
"The club will do what is within its powers to prevent this and will lodge a formal complaint to the authorities on Wednesday as there is now not the slightest doubt remaining about this tap-up attempt.
"Birmingham City stresses that this conduct, if condoned, will open up flood gates and set a very bad precedent."
The statement adds: "This is also an attack against the intelligence and integrity of not only the Villa fans, but most importantly our fans who are badly let down after having just been relegated.
"It is definitely a bitter pill to swallow for all fans concerned, both here and across the city.
"Never have the two clubs seen such a strong unity between their rival fans and this unity is based on humility and integrity and what is right and fair."
Blues have also hit out at a statement from the League Managers' Association which effectively gave its backing to McLeish.
The LMA statement criticised the sacking of Birmingham chief scout Paul Montgomery and McLeish not being "properly consulted" regarding player transfers.
Birmingham's statement continues: "The LMA's announcement is refuted as the club has a very rigid approval system where only players formally identified, and with approval forms signed and sanctioned by the manager personally are brought in.
"The chief scout was sacked only after Mr McLeish was informed who at that time had chosen not to comment.
"The allegations against the club, president Carson Yeung and the Board are frivolous excuses and without foundation.
"The matter is now with Birmingham City's lawyers and no further comments will be made."