McLeish vacated his place at the Championship side with two years to run on his contract, despite having lined up deals with a number of players for next season.
“I have just bought him three players of his choice,” Pannu told the Daily Express. “Having secured him the fourth one, Koevermans, I get an email. It speaks volumes.
“We spent £40m, supported McLeish all the way, and what we get is a relegation and a snub when Carson [Yeung] gave him unwavering support.”
McLeish took on the club in 2007. The Blues were relegated from the Premier League in 2008 but found their way back up the following year. They were relegated once again last season, during which they won the Carling Cup.
His resignation has sparked speculation that he may be walking into a managerial role elsewhere - notably City rivals Aston Villa - although Pannu stated that he opposes any such move.
“He is still our contracted staff and Birmingham City FC does not grant him any right to speak to any outsiders until the matter is resolved,” he said. “I’ll be meeting my legal team. It’s about time discipline is instilled into this game and I am not a person who gets pushed about.”