Bulgarian referees' chief quits
Kostadinov was criticised by CSKA Sofia following what they said was biased officiating during their 1-1 draw against champions Litex Lovech at the weekend.
CSKA officials accused Stefan Spasov of poor refereeing and called for the resignation of Kostadinov. Several other Bulgarian clubs have criticised the standard of refereeing this season.
The BFU will decide whether to accept Kostadinov's resignation next week.