Sun has suffered long-running financial problems
The European Commission (EC) has objected to Oracle's proposed takeover of Sun Microsystems, casting doubt on the $7.4bn (£4.4bn) deal.
The commission has issued a preliminary assessment raising concerns that the takeover could hurt competition in the computer database market.
Oracle said that it would "vigorously oppose" the EC objections.
The deal between business software firm Oracle and hardware and software maker Sun was first announced in April.
Last month, Sun said it would cut 3,000 jobs over the next year.
They are the latest in a series of cuts to be made by the firm, which has suffered prolonged financial woes.
Oracle maintains that there would be no conflict of interest and has promised not to sell off MySQL to get the deal approved.
It is looking to strengthen its position against IBM, which abandoned its own attempts to buy Sun earlier this year.