Ninety people from Diego Garcia are expected to arrive at Gatwick next month, West Sussex County Council says.
Diego Garcia was taken over for a military base in the 1970s
The arrivals would bring the total number in Crawley to 250 - the council says a further 3,000 may also come.
But the council, which the High Court has twice ordered to assist the islanders, says it could cost millions.
The UK passport holders left the island when it became a British army base in the 1970s. They are ineligible for UK benefits until settled in the country.
In July, a group of islanders failed in their bid to return to their Indian Ocean homeland.
Appeal judges also refused them permission to challenge a High Court ruling preventing further claims for damages.
Cllr Henry Smith, West Sussex County Council leader, said: "We have received a letter advising us that 45 Diego Garcians will be arriving at Gatwick on 8 October with a further 45 likely to arrive on October 20.
"The council has also been told in a letter from representatives of the Diego Garcians that there are a further 3,000 individuals 'waiting to come home'."
He said that the cost of looking after so many people before they can claim benefits would cost £18m.
The council, which wants the government to support the islanders, said it heard about the expected arrivals in a letter from the Brighton-based Rights Foundation.
Crawley MP, Laura Moffatt, said: "The original groups are all now settled, self-supporting, working and contributing to the economy.
"No British passport holders have a right to benefits and support if they return to or enter the UK for the first time. They must be able to support themselves from the start.
"If they have not been paying tax in this country, they must establish residency first.
"I continue to support their claim for compensation in relation to the original removal from Diego Garcia."