The League Managers' Association claim the 47-year-old was dismissed for negative comments made to the media about financial matters at Elland Road.
The intention to sue United was revealed last month after the LMA claimed Leeds had refused to enter into compensation talks.
A statement released by the LMA today said: "Proceedings will be lodged in the High Court today regarding Kevin Blackwell's claim against Leeds United Football Club following his dismissal in September.
"Obviously Kevin Blackwell and the LMA, who are supporting him in this matter, are very disappointed that they have been forced into this position by the fact that Leeds United have refused to put forward any proposals for compensation.
"The LMA has no further comment to make at this moment."
United have so far made no response to the LMA's announcement.