The new rule is slated to take effect on July 1.
The group of small islands, some 15 kilometers off Hoi An's coast, has been a favorite destination of many travelers.
Rapid increases in arrivals have put the islands, with a combined population of 2,500, under a lot of pressure, especially its environment and water resources, Hoi An's authorities said.
Official figures showed that more than 75,000 tourists visited Cu Lao Cham, which was recognized as a world biosphere by UNESCO in 2009, over the first four months this year.
That was nearly a three-fold increase from the same period last year, and during the national holiday on April 30 and May 1, up to 11,000 tourists came to the islands a day.
"If we continue to let tourists flock into Cu Lao Cham, we will likely lose the islands in a few years," Nguyen The Hung, vice chairman of Hoi An Town People's Committee, said in an interview with Thanh Nien.
Meanwhile, asked about travel companies' responses to the new rule, Dinh Thi Thu Thuy, chief of Hoi An's tourism division, said many of them had previously proposed authorities to take similar action, as "signs of exhaustion" already showed in the local boat station and services.
Authorities will collaborate with the businesses in controlling the situation so the number of visitors will always be within the limit, Thuy said.
In the meantime, she said, they have been promoting stay-over services on the islands, as statistics showed that less than 10% of tourists spent the night on Cu Lao Cham.
