One of Thailand’s most iconic islands looks set to close indefinitely because of damage caused by tourists.

Koh Tachai island, located off the coast of southern Thailand, is known for its idyllic shoreline, white sandy beaches and coral reefs - but from October 15 it will be shut to the public.

“The closure aims to ease negative effects on natural resources and the environment as a result of heavy tourism activities in the Similan National Park, a natural conservation area,” Department of National Parks, Wildlife and Plants Conservation Director General Tunya Netithammakul told The Bangkok Post.

“The measure is part of the masterplan for marine resource management in the Andaman Sea,” he said.

Experts said Koh Tachai can see up to 1,000 people on the beach at one time. The island’s capacity is 70.

"In fact, Koh Tachai is preserved as a primitive zone, not a tourist site. A beach on the island can hold up to 70 people. But sometimes the number of tourists was well over 1,000 on the beach, which was already crowded with food stalls and tour boats.

“This caused the island to quickly deteriorate. If it's not closed now, we'll lose Koh Tachai permanently," Kasetsart University Assistant Professor Thon Thamrongnawasarwat said.

Two deep dive sites in the area will remain open to divers and tourists.

The government is urging people to be on the lookout for dodgy tourism operators who may try and sell trips to Koh Tachai.

"We have to close it to allow the rehabilitation of the environment both on the island and in the sea without being disturbed by tourism activities before the damage is beyond repair," Mr Netithammakul said.