BBC south-east Asia correspondent Jonathan Head said the authorities were likely to move quickly to investigate the deaths to maintain confidence in a tourist industry which accounts for around a 10th of Thailand's GDP, and which has been hit hard by recent political turmoil.
Thailand attracts more than 850,000 British and Irish tourists each year.
Some 11 British nationals have been murdered in the country since 2009, according to the Foreign Office.
Official FCO advice warns that Western tourists have been victims of "vicious, unprovoked attacks by gangs" on the nearby island of Koh Phangan but there are no specific warnings for Koh Tao, which is a popular tourist destination.
TAT said Koh Tao and other beach destinations in Thailand remained safe for tourists.