After Brook and Khan failed to reach an agreement, Hearn has revealed that Brook will now defend his IBF welterweight belt against unbeaten American Spence at the end of May.
It means that the long-awaited bout between Brook and Khan may now never happen, but Hearn has insisted that the fight was "on a plate" for the Olympic silver medallist had he wanted to take it.
He told Sky Sports News: "The Amir Khan fight was there on a plate. Let me tell you, this fight is five times tougher than Amir Khan, and this isn't really about the money for Kell Brook.
"He has been called out that he would duck this fight against Errol Spence, particularly by Amir Khan, and he says no, that's not my style, I'm fighting Errol Spence in front of my people. I'm going to defend my belt with pride and honour."
Brook will be making his first appearance in the ring since being stopped in five rounds by Gennady Golovkin in September.