Frank Warren is suing former world boxing champion Ricky Burns for about £2m, saying the Scotsman was "tapped up" by rival promoter Eddie Hearn.
Burns, 31, told the High Court in London that he pulled out of an agreement with Warren's company because of unpaid fight purses.
But Warren said: "He got tapped up. That's the bottom line.
"Behind the scenes he got tapped up by another promoter. That's what this is all about and it's very disappointing."
Warren wants the money he says he lost when former WBO lightweight champion Burns signed with Hearn last year.
|Frank Warren, boxing promoter|
|"Up until this litigation, I quite liked him. Now I am very disappointed. Very, very disappointed."|
Burns, from Coatbridge, was under Warren's wing when he became WBO super featherweight champion in 2010.
He later moved up to lightweight and took the world champion's belt in that division too.
However, the relationship ended after a successful title defence in Glasgow was promoted by Hearn's Matchroom Sport Limited.
Burns subsequently signed an exclusive promotional deal with Hearn's company and signed up Alex Morrison as his sole manager.
In his High Court claim, Warren says the moves were in breach of a series of binding agreements he had signed with the boxer.
A promotional agreement gave his company, W. Promotions Ltd (WPL), exclusive rights to promote Burns' next three fights, he said.
Warren also claims a joint management agreement made him co-manager with Morrison, entitling him to a cut of the fighter's payments from 2010 to last year.
He says he is due about £90,000 in unpaid manager's commission and that his company is owed £1.8m in lost income.
Warren's barrister, Ian Mill QC, told Mr Justice Knowles that, at around the time the agreements were torn up, Burns was already in talks with Hearn.
"Documents disclosed by Mr Burns suggest that, at the relevant time, he was engaged in 'conversations' with Eddie Hearn at Matchroom, which led to a 'five-fight deal' with that company," he said.
Warren said he had done what he could to help Burns' career, often to his detriment as a promoter, such as pushing for a fight to be held in Scotland when overseas venues would have paid better.
Describing the boxer as having a "massive heart", Warren added: "Up until this litigation, I quite liked him. Now I am very disappointed. Very, very disappointed."
Burns disputes that Warren provided any more than promotion after 2010 and says the agreement with WPL was cancelled by non-payment of full fight fees.
Mark Simpson QC said the joint management agreement was not valid or enforceable and so Burns could not be held to it.
Even if he was entitled to some cut, Warren had waived his right by not taking it at the time, he said.
The hearing, expected to last several days, continues.