Sir Alex Ferguson has begun legal proceedings in his dispute with top Irish horse breeder and major Manchester United shareholder John Magnier.
The Old Trafford boss is claiming ownership rights over the highly successful stallion Rock of Gibraltar.
Ferguson claims Magnier gave him a 50% share of the horse, which won a record seven successive Group One races.
He says this entitles him to half the stud fees - proceeds from Rock of Gibraltar's first season are expected to top £12m.
But Ferguson, who has now issued a writ, says he has been offered only a fraction of the horse's stud value.
Magnier's Coolmore Stud is prepared to fight Ferguson all the way and released a statement of intent through public relations firm Murray Consultants.
It said: "In response to the lodging of legal proceedings against our clients by Sir Alex Ferguson, our clients consider them to be without merit, and they will be vigorously contested."
The bad feeling between the two men grew after Magnier and his business partner JP McManus, via The Cubic Expression Company Limited, increased their shareholding in Manchester United to just over 23% last month.
The move fuelled speculation that the pair were preparing a takeover of the Premiership champions.
That would leave Ferguson's future uncertain, given the intensity of the dispute.