BORACAY, Philippines — A Philippine island that became famous for the white sand beaches and coral reefs that made it a haven for travelers, and then for the rapid development and pollution that threatened its idyllic shores, has been ordered closed to tourists for six months.

President Rodrigo Duterte — who has said that inadequate sewage treatment on the island, Boracay, has turned its beaches into a “cesspool” — directed that it be closed to tourism starting on April 26, his spokesman, Harry Roque, said Wednesday.

That decision threatens the livelihoods of thousands of people on Boracay, a speck of an island in the central Philippines that has become a travel destination to rival Phuket, in Thailand, and Bali, in Indonesia.

Mr. Duterte has said that Boracay must be cleaned up, but officials have given no indication that there is a plan to do so. Yet last month, the government signed an agreement with Galaxy Entertainment, a Macau-based casino operator, and a Philippine partner to begin construction next year on a $500 million beachfront casino that would add to the burdens on the island’s infrastructure.