Authorities have stepped up moves to house the Rohingya on the island in the Bay of Bengal since a new surge which now totals 436,000 refugees started arriving on August 25.
UN High Commissioner for Refugees Filippo Grandi said Prime Minister Sheikh Hasina had mentioned the relocation plan when they met in July. There were already 300,000 Rohingya in camps near the border at Cox's Bazar before the latest influx started.
But he insisted that any move from the camps to Bhashan Char island-- also known as Thengar Char -- "has to be voluntary on the part of the refugees".
"We cannot force people to go to the place. So the option for the medium term, let's say -- I don't want to talk about long-term -- has to be also something that is acceptable to the people that go there," he said.
"Otherwise it won't work. Otherwise people won't go."
The United Nations has praised Bangladesh for taking in the Rohingya, who fled a military crackdown in Myanmar. It has appealed for international help for the authorities.
"It is good to think ahead. These people (Rohingya) may not be able to go back very quickly and especially now the population has now doubled," Grandi told a Dhaka press briefing.
The UNHCR chief said his agency was ready to help the island plan with a "technical study of the options.