The Republican People's Party's (CHP) appeal against the April 16 referendum results was rejected by the Turkish administrative court based in Ankara.
Turkey's main opposition party will appeal to the European Court of Human Rights to challenge the acceptance of unstamped ballots by the electoral board in the April 16 referendum on changing the system of governance, a party spokeswoman said on Wednesday.
"I want to reiterate once again that we will take the referendum result to the ECHR," Republican People's Party (CHP) spokeswoman Selin Sayek Boke told reporters in Ankara.
Turkey's Council of State, the highest administrative court, on Tuesday rejected the CHP's appeal on the referendum, citing a lack of jurisdiction.
Earlier on April 19, the country's Supreme Electoral Board (YSK) rejected appeals from the main opposition parties to annul the referendum that will change Turkey's system of governance from a parliamentary to a presidential system.
The CHP had called on the electoral board to repeal the April 16 vote, which was won by the "Yes" camp, because unstamped ballot papers were included in the vote count.
"CHP, HDP and Vatan Party appeals regarding the April 16 referendum were discussed separately, and as a result of evaluations, the appeals were rejected with 10 votes against and one vote in favour," the electoral board said.
The CHP responded by saying it would employ all legal ways to appeal the referendum.