The Iraqi government has said it would reject any unilateral move by Kurdish regional authorities to press for independence, according to a spokesman in the capital, Baghdad.
The statement on Friday comes two days after Masoud Barzani, president of the Kurdistan Regional Government (KRG), announced that a referendum on Kurdish independence would be held on September 25.
"No party can, on its own, decide the fate of Iraq, in isolation from the other parties," Saad al-Haddithi, Iraqi government spokesman, said in a statement.
"Iraq is constitutionally a democratic, federal country with full sovereignty ... Any measure from any side in Iraq should be based on the constitution," Haddithi said.
Iraqi Prime Minister Haider al-Abadi had said in April that he respected the Kurdish right to vote on independence, but he did not think the timing was right for the move.
The referendum on whether to secede from Iraq is planned to be held in the three governorates that make up the Kurdish region, and in the areas that are disputed by the Kurdish and Iraqi governments but are currently under Kurdish military control.
The disputed areas include swaths of northern territory that are claimed by both Kurdish Iraq and Baghdad, including the key oil-rich province of Kirkuk.
Opposition in Baghdad to Kurdish Iraq becoming independent would become even greater if the region tried to take disputed territory along with it.
Turkey also came out against the referendum on Friday, calling the plan a "terrible mistake".
"The maintenance of Iraq's territorial integrity and political unit is one of the fundamental principles of Turkey's Iraq policy," the foreign ministry said in a statement.
States that neighbour Iraq, including Turkey, as well as Syria and Iran, and which all have large and sometimes restive Kurdish populations, have in the past resisted moves towards Kurdish independence.