Earlier this week Klaasen left the British Darts Organisation to join the Professional Darts Corporation (PDC).
But the BDO claims the 22-year-old is under contract to it for another year, and spokesman Robert Holmes told BBC Sport: “We mean business over this.
“That hasn’t always been the case in the past but it’s going to be from now on – we are consulting our lawyers.”
Klaasen moved to the PDC along with fellow Dutch players Michael van Gerwen, the 2006 World Masters winner, and Edwin van der Voort.
Speculation has continued over England’s Mervyn King following suit, while Darryl Fitton also claims to have had an offer from the PDC.
Holmes claimed: “The PDC is a business, while the BDO run darts as a proper sport.
“It has a system that looks after players of all abilities and has created all the top names in darts. We don’t buy and sell players, we create them.”
Holmes said the BDO championships, in their 30th year on BBC TV in 2007, builds players’ profiles – this year’s final was seen by a peak audience of five million.
He added: “Players leaving the BDO will always be replaced through the system, and we will usually wish them luck.
“But where necessary, as in the case of Jelle Klaasen, it’s our duty to remind them of their contractual obligations.”
The BDO claims Klaasen is midway through a two-year deal to play at the Lakeside where he became the youngest-ever BDO world title winner in 2006.
He and his compatriots have followed four-time Lakeside champion Raymond Van Barneveld, who defected after losing to Klaasen last year.
He beat Phil Taylor to win the PDC’s version of the title on New Year’s Day.
(From BBC Sport)