Italian businessman Massimo Cellino has been disqualified from taking over Leeds by the Football League's board.
The League said his recent conviction for a tax offence in Italy constituted "a disqualifying condition under its owners' and directors' test".
The decision leaves Leeds in a state of limbo over its future ownership - Cellino has already paid more than £2million into the club.
The decision was taken at a meeting of the league's board on Sunday evening which considered Cellino's eligibility in the wake of his conviction in Sardinia.
The league said in a statement: "Mr Cellino was recently found guilty beyond reasonable doubt by a Court in Sardinia of an offence under Italian tax legislation relating to the non-payment of import duties on a boat. This resulted in a fine of 600,000 euros (£500,800), an order for the payment of trial costs and the confiscation of the boat in question.
"Having fully considered the matter, the board agreed unanimously that the decision of the Italian court does constitute a disqualifying condition under its owners' and directors' test.
"The relevant disqualifying condition being that Massimo Cellino has been convicted of an offence involving acts that would reasonably be considered to be dishonest."
Cellino is entitled to lodge an appeal against the decision within 14 days.
The statement added: "In such circumstances, the League would seek to expedite the process to deliver certainty to all parties in the shortest possible timeframe."
The club's whole future is now shrouded in uncertainty. Current owners Gulf Finance House (GFH), a bank in Bahrain, had signed a deal for Cellino, the owner of Serie A club Cagliari, to buy 75% of Leeds.
Leeds have also being struggling with financial problems though GFH has insisted the club will not go into administration again.
Cellino's lawyer had argued that as he is appealing against the Italian court's decision, he should be viewed as innocent until that final sentence is passed.
The league's board disagreed however, say in its decision that "if a competent Italian court (such as the Sardinian court in this case) finds a defendant guilty of a crime and specifies the sanctions to be imposed upon him then under Italian law the defendant is considered to have been convicted of a crime even if the judgement is not final because the defendant has the right to appeal".
GFH said it had "no comment at this stage" on the decision.