Harrison won Prizefighter in October
Audley Harrison's European title clash with Albert Sosnowski is off because the Pole has been offered a world title shot against Vitali Klitschko instead.
Former Olympic champion Harrison was scheduled to challenge Sosnowski at London's Alexandra Palace on 9 April.
But the 31-year-old Sosnowski will now fight for Klitschko's WBC heavyweight belt in Gelsenkirchen on 29 May.
The April card will still go ahead, with Londoner Harrison, 38, taking on an as-yet unnamed opponent.
He last fought when winning the Prizefighter heavyweight tournament last October.
After 10 months of inactivity, Harrison knocked out Scott Belshaw in two rounds in the final to earn £32,000 in prize money and a contract with Matchroom Sport promoter Barry Hearn.
He had hoped victory over Sosnowski would lead to a shot at Vitali Klitschko or his younger brother Wladimir, owner of the IBF and WBO belts, but now Harrison will have to consider other options.
Sosnowski, who is based in Brentwood, Essex, is best known for a stoppage win over Danny Williams in 2008, while he claimed the European title with an emphatic points victory over Italian Paolo Vidoz in December.
Sosnowski said: "It's a dream come true. Because of the Harrison fight I am already in training and suddenly here comes the once-in-a-lifetime chance to fight Klitschko.
"For me, Klitschko is already over the hill. I know that I am the underdog for everybody, but I tell you, Klitschko will regret that he picked me.
"Many Polish and Germans with Polish roots live in the Gelsenkirchen area and I hope that they will support me. I will be the first Polish heavyweight champion of the world."
Klitschko said: "I saw Sosnowski's fights on DVD and must admit that he is a very experienced, quick and tough opponent.
"He will do anything to get my belt but I promise that this is not going to happen."
Meanwhile, Klitschko said he would flip a coin with his brother to decide who will fight Britain's WBA champion David Haye, if Klitschko defeats Sosnowski and Haye defeats John Ruiz on 3 April.
Haye's plans to fight both Ukrainian brothers have fallen through before, but Klitschko appears keen to make the fight happen later this year.
"I told my brother, 'I want to fight Haye', but he said no, he wants to fight him," said Klitschko.
"For him it is personal, but for me I want the title. My brother already has three [Wladimir is also the IBO title-holder] but I have to clear it with him. I hope he listens to his older brother.
"I'll tell him it's easier to beat me, because I'm older and I don't have as much energy as my brother. I keep my fingers crossed that Haye beats Ruiz. Not because I like him, but I want his title and to shut his mouth."