European regulators threw up a road block Thursday for Oracle’s planned $7.4 billion acquisition of Sun Microsystems, saying they would investigate whether the deal would restrict competition in the already “highly concentrated” market for databases.

The deal had been backed by Sun shareholders in July and by the U.S. Justice Department in August, leaving EU consent as the last obstacle to its conclusion. Analysts had expected it to pass muster in Brussels with little fanfare.

But Neelie Kroes, the EU competition commissioner, announced instead that an in-depth investigation would be opened...