April 30, 2017 0114 GMT
"We are filing an appeal to the council of state today demanding the cancellation of the YSK decision to accept unstamped ballots," CHP Deputy Chairman Bulent Tezcan said in a statement, Reuters reported.
Turkey's Council of State is the high judicial body handling all complaints and appeals against state and public institutions. The CHP and a pro-Kurdish opposition party had pledged to take legal action against what they said were irregularities in Sunday's referendum.
Tezcan also said the CHP would demand that the official results of the referendum be postponed until the case is resolved. The YSK had said on Sunday it would announce final results 11-12 days after the vote.