Two backpackers drowned after going swimming in the rough sea allegedly following an all-night drinking session with friends in Thailand.

Tourists Anna Carreras, 21, from Spain, and Rahul Ramkurrun, 22, from France, had allegedly stayed all night partying with other young holidaymakers on Koh Chang island in Trat province.

However, they reportedly told hotel staff at around 5 am they were going to the beach to watch the sunrise on early Sunday morning.

The pair ignored a red flag warning tourists of high waves and strong currents due to the annual rainy season and went swimming in the sea. The rough waters churned up by the summer storm then swept them under the surface.

Local council boss Termsak Sertsri said the tourists earlier told staff at the Lonely Beach hotel where they were staying that they were going to watch the sunrise, but instead went to the beach.

He said: 'The tourists did not tell the staff they were going into the sea. They would have warned them not to. There are also red flags on the beach right now because the waves are very powerful.'

A resort employee told local media: 'The night before I had seen the female tourist and friends. There were six foreigners, all of whom were friends. They drank and partied at a restaurant near their accommodation, and later, around 5 am, the tourists went swimming along the beach while the waves were strong.

'At that time, the resort staff did not see the incident. Then some of the foreign friends who were swimming together with them in the sea saw that two of them had been washed away from the shore. They shouted to the resort staff for help.'

Locals raised the alarm and rescue teams on jet skis scoured the treacherous waters around 400 metres from the shore for signs of the missing victims.

They eventually found Anna at about 8:10 am local time floating some 300 metres south of the beach. She was pulled to dry land but was lifeless.

Officers continued searching for three more hours before poor weather conditions forced them to temporarily call off the operations.

Rahul's lifeless body was finally found at 9:45 am on July 11.

Ko Chang National Park chief Dusit Samutrakpong warned tourists to be careful when swimming on beaches, especially if red flags have been hoisted.

He said: 'The sea is strong during the rainy season so people should not go swimming. It is calmer at the start of the year but now there are storms which make it unpredictable.'