A Turkish court has declined to hear an appeal by the main opposition party challenging the acceptance of unstamped ballots in the referendum to expand the powers of the presidency.

The Council of State, Turkey's high judicial body handling complaints and appeals against state and public institutions, said it had no jurisdiction in the case, state-run Anadolu news agency said on Tuesday.

The court rejected the opposition Republican People's Party's (CHP) petition by majority vote, saying it could not rule on the decision by the High Electoral Board (YSK) to accept ballots without official stamps because it was "not an administrative procedure", reported the agency.

The Council of State was not immediately available for comment.

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The opposition argues that the rule change - made on the day of the vote - opened the way for fraud. The CHP, which is led by Kemal Kilicdaroglu, had launched a failed bid to annul the referendum last week.

The results are due to be announced 11 to 12 days after the April 16 referendum, the YSK has said.

Preliminary results put the "Yes" vote at 51.4 percent.

Held under a state of emergency in place since a failed coup last July, the referendum was criticised by European election observers, who said the decision to allow unstamped ballot papers to be counted had removed a main safeguard against voting fraud.

Turkish President Recep Tayyip Erdogan and government ministers have rejected criticism of the vote as politically motivated, and the YSK last week dismissed challenges by the CHP and two other opposition parties.

Bekir Bozdag, Turkey's justice minister, said on Saturday that decisions made by the YSK could neither be challenged in the Constitutional Court nor the Council of State.

Levent Gok, CHP's parliamentary group leader, hit back, accusing Bozdag of giving "instructions" to the judges before they had made a decision.

Turkey will implement an executive presidency from November 2019, axing the role of prime minister and granting the presidency sweeping powers.