Canadian sources have named the women as Audrey Belanger, 20, and her sister Noemi, 26. Officials offered condolences to family and friends and said Canadian envoys are working with Thai authorities on an investigation.
PHUKET: Two Canadian sisters aged 20 and 26 have been found dead in a room at a hotel on the popular holiday island of Phi Phi, reviving the riddle of two mysterious deaths of young women in 2009.
At this stage the latest tourist deaths also remain a mystery, with a Phi Phi policeman telling Phuketwan there were no signs of violence.
The bodies, found at the Phi Phi Palms Residence, are now at Krabi Hospital, a nurse told Phuketwan this afternoon. A team of pathologists is likely to autopsy them as soon as possible.
Police believe the sisters may have been poisoned. As with a similar case on Phi Phi three years ago, a maid opened a door to find the sisters dead.
The sisters - reported this evening to be from Quebec - checked in on Tuesday. Lieutenant Wisawa Senghar, who is based on Phi Phi, told Phuketwan ''The sisters initially planned to only stay one night but told a receptionist after staying on Tuesday that they would like to stay Wednesday night as well.''
He said they stayed in a ground-floor room at the small resort.
The maid knocked on Thursday to clean the room, but there was no response. Only yesterday was a master key used to open the door.
As with the earlier deaths of an American woman and a Norwegian woman, there were signs of vomiting in the room.
''It really is difficult to speculate what may have killed them,'' Lieutenant Wisawa said. ''We are waiting for Canadian Embassy officials to come to check out the room.''
Nurse Anangkana Choisrinal, at Krabi Hospital, said today: ''I have never seen a case like it. We have no idea as yet what killed the women.
''They had been dead between 12 and 20 hours when they were found. They both have a mysterious rash.''
Nurse Anangkana said Krabi Hospital had yet to hear from the Canadian Embassy.
Flamboyant spiky-haired Dr Pornthip Rojanasunand, considered by many to be Thailand's leading pathologist, told Phuketwan today that she had yet to be contacted about the case.
''There must be a reason why they died,'' she said. ''I feel certain a cause will be found.''
Phi Phi, like neigboring Phuket, is a popular Thai island destination with fun-seeking 20-somethings.
The island has a reputation for tropical night life and binge drinking - and it was recently chosen as the world's top must see island by TripAdvisor, the internet travel site. Young people on the island drink alcohol and fruit juice mixed in ''buckets'' of various sizes.
Phi Phi visitors tour around, enjoying the bars and topping up their Phi Phi ''buckets'' all night long. Bars close at 1am in most of Thailand and are supposed to close at 2am in Phuket and other tourist destinations but the bars of Phi Phi are reported to stay open until dawn.
By day, the island is more sedate as visitors come from Phuket and Krabi for snorkelling and diving. The surrounding landscape, rising up sharply from the sea in limestone formations known as karsts, is spectacular.
Maya Bay nearby was the location for the shooting of many scenes from the cult movie 'The Beach.'
About 80 or 90 minutes from Phuket by ferry, Phi Phi was also the scene of the still unexplained deaths of two young women in May, 2009, at a guesthouse.
The mystery of what killed the women - who died after falling sick in adjoining rooms - has never been solved.
American Jill St Onge, 27, was in one room with her boyfriend Ryan Kells, while Norwegian Julie Michelle Bergheim, 22, and her friend ''Karina'' signed in for the room next door.
Within hours, St Onge and Bergheim were dead, and Kells and ''Karina'' narrowly avoided the same fate.
The Laleena guesthouse has since changed its name but the mystery lives on.
Despite pathologists and chemists in Norway and the US trying to solve the mystery, no cause of the Laleena deaths has ever been determined.
Although the latest tragedy is still unfolding, the fact that the 2009 deaths remain unsolved and that the victims were also young women is likely to lead to intense media coverage.
Last year in Thailand, there was yet another riddle - the mystery of what killed 23-year-old New Zealand woman Sarah Carter and several other tourists in Chiang Mai and the city's Downtown Inn.
The cause of those deaths has never been fully resolved. The demolition of the Downtown Inn has since been ordered.
Online, the resort where the Canadian women died is described as follows:
PHI PHI TWIN PALMS BUNGALOW & PHI PHI PALMS RESIDENCE are located at Loh Dalam Bay in Phi Phi Island. One of the most popular destinations in the South of Thailand. It is just 45 kms to the southeast of Phuket and 45 kms to the southwest of Krabi. Fine, white sandy beaches and breathtaking rock formations rising from the vivid turquoise waters that surround the islands will instantly confirm you've arrived in paradise.
Our Hotel is run by a local Phi Phi island Family, dedicated to tending to you every need in a warm and friendly way.