CLEVELAND, Ohio (AFP) – They torched his jersey in the streets but nobody felt more burned about LeBron James' decision to bolt Cleveland for Miami than his former boss, Cavaliers owner Dan Gilbert.

Gilbert levelled his guns at James Thursday in a blistering verbal attack on the reigning NBA MVP for signing as a free agent with the Heat.

He described James as "selfish" and said his actions were a "cowardly betrayal" of his hometown.

"You simply don't deserve this kind of cowardly betrayal," Gilbert said in the letter to the Cleveland fans. "You have given so much and deserve so much more. In the meantime, I want to make one statement to you tonight:

"I personally guarantee that the Cleveland Cavaliers will win an NBA championship before the self-titled former 'King' wins one. You can take it to the bank.

Gilbert said he couldn't believe that one of Cleveland's native sons would let down the city like this.

"If you thought we were motivated before tonight to bring the hardware to Cleveland, I can tell you that this shameful display of selfishness and betrayal by one of our very own has shifted our 'motivation' to previously unknown and previously never experienced levels."

Not long after James' decision to join Dwyane Wade and Chris Bosh with in Florida was announced, one of his jerseys was shown being burned in the city's streets.

"This shocking act of disloyalty from our home grown 'chosen one' sends the exact opposite lesson of what we would want our children to learn.

"And 'who' we would want them to grow-up to become," Gilbert wrote.

"But the good news is that this heartless and callous action can only serve as the antidote to the so-called "curse" on Cleveland, Ohio."

James says he felt "awful" about leaving Cleveland but he believes his chances of winning an NBA title are better in Miami.

"Winning is a huge thing for me," said James, who added he left more than 30 million dollars on the table by not re-signing with Cleveland.