Erdogan signed a membership document to rejoin the Islamic-rooted Justice and Development Party (AKP) that he co-founded, at a ceremony at its headquarters, an AFP photographer said.
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Party officials then burst into thunderous applause and sang the national anthem.
Erdogan was accompanied by his wife Emine and was sat next to the AKP chairman and Prime Minister Binali Yildirim.
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The president will likely be reinstalled as party chairman on May 21 at an extraordinary AKP congress and replace Yildirim, who is set to stay on as premier.
After Erdogan signed the document, Yildirim told the president he had come back "home", adding: "Welcome, you have honoured (us)."
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