The country's Supreme Election Council (YSK) announced its 7-4 decision on Monday due to electoral fraud allegations after Turkish President Recep Tayyip Erdogan's party lost the razor-thin race margin.
The opposition Republican People's Party (CHP) won control of Istanbul and capital city Ankara in the March 31 local elections, dealing a blow to Erdogan's Justice and Development Party, known as AKP.
But in Istanbul, the victory was a narrow one, with CHP at 48.79% of the vote -- just ahead of AKP's 48.51%. According to Anadolu, the re-run will occur on June 23 and Ekrem Imamoglu, Istanbul's new CHP mayor, had his mayoral certificate canceled by YSK on Monday.
Turkey's opposition denounced Monday's ruling. "It is illegal to win against the [AKP] Party," CHP's Deputy Chairman Onursal Adiguzel said on Twitter, according to Reuters.
"This system that overrules the will of the people and disregards the law is neither democratic nor legitimate".
The ruling AKP had appealed to the board in April to annul the result in Istanbul and hold a re-vote after raising questions about potential voter fraud. Claiming "organized irregularities," AKP Deputy Chairman Ali İhsan Yavuz said there was "only one body to clear the queries.
It is Supreme Election Council. Their decisions are binding for all of us." Reuters reports that the YSK had already ordered partial and full recounts across Istanbul.
Imamoglu took office in April as the recounts were completed.
Before March's vote, the AKP and its predecessor had consistently won in Istanbul's local elections since the 1990s, when Erdogan launched his mayor's political career.
Erdogan served as the face of AKP's local election campaigns this year, and the elections were widely seen as a referendum on his government.