Joe Calzaghe broke a verbal contract with promoter Frank Warren which could cost Warren at least £1m, according to a High Court claim.
The claim on behalf of Warren's company Sports Network says it was agreed Warren would still promote Calzaghe after he fought Bernard Hopkins.
But after beating Hopkins in April, Calzaghe said in June he was ending his 12-year association with Warren.
Warren lodged his claim three days later. Calzaghe says he will fight it.
"Joe, are you giving me your word that I will continue promoting you? "
In his South Wales Argus newspaper column on Saturday, the Newbridge boxer said he was confident of success in his legal battle with Warren, and insisted he would counter-claim, alleging he was owed £2m from the Hopkins fight.
However, a counter-claim has yet to be filed.
The High Court claim, seen by the BBC on Thursday, gives the promoter's version of a meeting in January between Calzaghe, his father and trainer Enzo, Warren and a Warren representative called Ed Simons.
According to the claim, Calzaghe's solicitor was excluded from the meeting at Warren's request.
Warren claims a verbal agreement was struck which would allow him to promote all future fights after Hopkins, and Warren would take 20% of Calzaghe's income from the Hopkins fight.
The High Court papers quote the words allegedly exchanged between the pair:
"Mr Warren: 'Joe, are you giving me your word that I will continue promoting you? Look me in the eye and give me your word.'
"The defendant (Calzaghe): 'I agree Frank there will be no problems moving forward with you'."
Warren's claim said the purse for fights after Hopkins would be "negotiated in good faith".
The court papers go on to say Calzaghe beat Hopkins on a split decision in April, but by mid-June, the undefeated champion was not responding to Warren's phone calls or letter.
After Calzaghe announced he intended to manage himself and fight Roy Jones Jr, Warren lodged his claim, estimating his loss for not promoting the Jones Jr fight to be "in excess of £1m".
Calzaghe later postponed that New York fight because of an injury to his right hand.
Warren is suing for damages for breach of contract, loss of the money he would have received from promoting the Jones Jr fight and "loss of enhancement of reputation and publicity".
Calzaghe has been contacted for his response.
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