Golfer Rory McIlroy has begun a legal action against his former management company, Horizon Sports Management.
The action was admitted into the fast-track commercial division of the High Court this afternoon.
The 24-year-old, who has an address in Monaco in Monte Carlo, wants to have his representation agreements with Horizon declared void.
The court was told the case would involve a range of complex issues and that Horizon intends to issue a counter-claim against Mr McIlroy.
In documents submitted to the court, lawyers for Mr McIlroy said Horizon began acting for the golfer in October 2011, by which time he had become one of the world's leading golfers.
The court was told he was the most sought-after players in professional golf from an agent's perspective, because of his youth, his positive image, and the likelihood he would have a long and successful career lasting 20 to 25 years.
The court was told Mr McIlroy had full trust in Horizon and that Horizon was aware of his lack of expertise in business affairs.
Mr McIlroy alleges that he signed an agreement with Horizon in late 2011 without receiving substantive independent legal advice.
He says this agreement appointed Horizon for a three-year term, which was too long, and charged him too much.
He alleges the agreement was heavily weighted in favour of Horizon and detrimental to his interests. He claims they took advantage of his lack of knowledge and experience of commercial transactions.
The agreement charged him 5% of prize-winnings from golf events and 20% of pre-tax sums from sponsorship and appearance fees.
He claims extensions to the agreement, signed in March last year, were very much adverse to his interests. The extension agreement extended the representation period from three years to six years and provided for Horizon to continue receiving commission in relation to a $20m (€14.7m) per annum sponsorship deal with Nike.
The court heard he had paid Horizon $6.8m (€5m) to date.
In a statement, Horizon said it was disappointed with the decision to begin legal action. It said under its management Rory McIlroy had signed some of the most lucrative endorsement contracts in sports history and was one of the world's most highly remunerated sports people.
It said it would vigorously and comprehensively defend the claim and had confidence in its position.
Horizon said it would always be proud of what they achieved together and wished Rory McIlroy well in his future golf career.