Tourism operators are concerned the oil slick off Rayong province will chase away tourists from the Eastern Seaboard if the problem is not resolved within the week.
Association of Domestic Travel president Yutthachai Soonthornratanawet said the oil leak will damage the tourism industry.
He said much of the tourism business is about to shift from the Andaman coastline to the Gulf of Thailand as visitors flee harsh seasonal weather.
Tourist authorities have promoted tourism in the East and the number of visitors to the region surged by as much as 50% last year.
Operators are concerned that reports by overseas news agencies will cause foreign tour agencies to book alternative holidays for their customers.
Travellers must be informed that it is still safe to visit, Mr Yutthachai said.
He said the authorities should provide information on the affected areas and should find a solution quickly.
If the authorities drag their feet, other tourist provinces in the Gulf could also lose business, he said.
Rayong province is one of the key tourist destinations in the East.
Koh Samet is a popular attraction among foreign and domestic tourists from Pattaya.
In 2012, tourism made about 16 billion baht for Rayong.
Koh Samet, with its 40 hotels, contributed 40% of the total revenue.
Chairat Trirattanajarasporn, chairman of the Rayong Tourist Association, said a large number of Chinese and Russian tourists are visiting Koh Samet.
The announcement that the area is being closed temporarily while it is being cleaned will cause some people to postpone their visits, he said.
Previously, the association projected that tourism revenue in Rayong would grow by 10-15% this year, but the oil leak will probably limit growth.
Mr Chairat said tourism operators are concerned about the environmental affects of the oil slick and fear it will not be properly controlled.
Chuchart Oncharoen, director of the Tourism Authority of Thailand, Rayong Office, said Koh Samet is the most important of Rayong's tourism destinations.
He said that so far the island has only been slightly affected by the spill as the oil slick has reached only Ao Phrao and the other 14 bays of Samet island remain intact. Mr Chuchart said there are three resort hotels along Ao Phrao with a total of about 100 rooms. Only seven bookings have been cancelled so far at Ao Phrao Resort, he said.