ISTANBUL — Ekrem Imamoglu, Istanbul’s new mayor, has squared off against his powerful rival President Recep Tayyip Erdogan over government plans to build a canal that would render half the city into an island, warning the project will wreak ecological and economic devastation.

Canal Istanbul, an artificial waterway that would link the Black Sea with the Sea of Marmara to bypass the vulnerable Bosporus Strait, is Erdogan’s signature project, which he acknowledged was “crazy” when he unveiled it in 2011. Government estimates of the cost are between $13 billion and $20 billion, while some economists estimate the final price tag is likely to be twice as high.

Imamoglu, who has emerged as a potential challenger to Erdogan after winning control of Istanbul’s city hall earlier this year, formally withdrew the municipality from a cooperation agreement on Canal Istanbul last week, calling it a “a disaster project … that will massacre the environment.”

It will destroy water reservoirs, aquifers and a catchment system that together supply drinking water to almost a third of Istanbul’s 16 million residents, Imamoglu said at a news conference on Dec. 25. Excavating an estimated 2.5 billion cubic meters of earth will increase the risk and severity of earthquakes in Istanbul, which sits on active fault lines, he said.