The New York Post reported that the former presidential candidate and social activist threatened to lead a protest at the Democratic National Committee's Washington headquarters if the group committed the "grave injustice" of seating the delegates.
Sharpton's comments in a letter to DNC Chairman Howard Dean come on the heels of NAACP leader Julian Bond's urging of Democratic Party officials to seat the delegates or run the risk of "disenfranchising" minority voters.
Florida and Michigan were stripped of their delegates to the Democratic convention after violating national party rules by setting their primaries before Feb. 5.
"I firmly believe that changing the rules now and seating delegates from Florida and Michigan at this point would not only violate the Democratic Party's rules of fairness, but also would be a grave injustice," Sharpton wrote.
Sharpton said his Harlem-based National Action Network is considering a march on DNC headquarters.
"This smacks of the same stuff we accused the Republicans of in Florida in 2000 ... changing the rules," he was quoted as saying.