(Bloomberg)—Turkey’s main opposition leader told potential backers of a canal project in Istanbul that he would not welcome them if President Recep Tayyip Erdogan loses the next election.

On Friday, the transport minister said the project will cost around $15 billion and the government plans to break ground by the end of June. The 45-kilometer (28-mile) waterway would be the second for the country to link the Black Sea with the Sea of Marmara, which Erdogan dubbed as his “crazy project” when it was announced a decade ago.

“If a foreign country enters that tender, we will keep our distance from that country when we come to power,” Kemal Kilicdaroglu, chairman of the Republican People’s Party, or CHP, told news site T24. “We will absolutely not pay them.”

He also warned local banks against extending loans to the project. Turkey’s next general elections are scheduled to be held in 2023.

Critics say they’re concerned the venture will impact an international agreement regulating the traffic through the Bosporus and the Dardanelles straits and meant to ensure stability in the Black Sea region.

Erdogan has said Turkey won’t exit the treaty but sees Canal Istanbul as an alternative to it as the project will enhance the government’s control of shipping to and from the Black Sea.

Opposition parties say commercial ships can’t be forced to take the alternative route and the canal would hit taxpayers and the environment.

The canal would “annihilate” water resources for Istanbul’s 16 million residents, ruin the province’s nature beyond repair and make it uninhabitable, according to Ekrem Imamoglu, the mayor of the nation’s largest city. Erdogan’s government has brushed off such concerns.