Amelia Bambridge, from Worthing, West Sussex, was last seen on the island of Koh Rong in Cambodia late on October 23.She was reported missing after she failed to check out of her hotel on time and staff at Police Beach, a private venue on the island, found her purple rucksack with her purse, phone and bank cards inside the following morning.
Members of Ms Bambridge's family have travelled to the island to join in the search for the 21-year-old.
On Wednesday the police chief for the area where Ms Bambridge was last seen said he believed she has drowned.
Major general Chuon Narin, police chief for Preah Sihanouk province, said that searches of Koh Rong island and in the Gulf of Thailand failed to provide any information about her disappearance.
However, he said that land and sea searches will continue until she is found.
About 150 volunteers including divers, navy personnel and visitors as well as Cambodian police are involved in searches for the missing backpacker.
Six men who work at restaurants, hotels and guesthouses on the island were questioned and released without charge on Tuesday evening.
Officials said the men were brought into custody because of complaints by Western tourists that they had acted badly towards visiting women.
On Wednesday, Ms Bambridge's family criticised the Foreign Office for their lack of support in a petition.