Motherwell say they pursued the claim "firmly but courteously" but failed to receive a "business-like response".
No comment has yet been made by Cardiff City.
Fir Park chief executive Leann Dempster said in a statement:
"It is critical that clubs work constructively with each other to ensure we can manage a very difficult financial climate for the game.
"We have given Cardiff every chance to do the right thing.
"The board now believe that Cardiff's position is untenable given their activity in the transfer market acquiring Jason Koumas, Tom Heaton, Danny Drinkwater, Seyi Olofinjana, Martin John and, today, the loan signing of Manchester City's Craig Bellamy.
"Given all these circumstances and with legal and moral weight behind us, Motherwell FC have no option but to immediately pursue all legal routes to ensure this significant sum paid to us as quickly as possible.
"These may include issuing a winding-up order, arresting assets or physically sending bailiffs to their ground on match day to take possession of cash.
"Every option will be fully considered and implemented as soon as possible."