A British woman is missing after the scuba diving boat on which she was a passenger capsized off a Thai island.

Millie Young, 57, from Brighton, and a member of the local staff are still unaccounted for since the vessel sank on Saturday morning, with rescuers growing increasingly concerned for her life.

According to local media reports, the boat — the Reggae Queen — hit rough waters around the Surin Islands archipelago in southern Thailand, a tourist hotspot for diving.

The diving boat had 18 people on board when it began to take on water, the Bangkok Post reported, with 16 of them quickly rescued and taken to a nearby pier.

Her friends Jo and Phillip Degregorio and their two children Oska and Daisy, also British, were among those rescued when they were pulled aboard a passing fishing trawler.

Officials said that a Mayday call was made shortly before 9am and the boat sank shortly after. It was battered by large waves around four nautical miles away from the archipelago and quickly began to take on water.

As the search for the two missing women stretched into a second day, the Thai navy had people scouring the area by land, sea and air.

Supoj Rodruang Na Nongkhai, the governor of Phang Nga province, said he had instructed all boats in the area to look out for the two missing people in the Andaman Sea region.

He said that rescue teams were searching in the direction of the sea currents, which are believed to have carried the missing pair in a northwest direction.

The incident has brought renewed attention to safety standards in the popular tourist destination.

A day before the accident that swept Mrs Young away, a group of foreign tourists had to be rescued from a ferry that hit high waves and sank near Koh Tao.

A group of Russian tourists were also rescued after their boat capsized in Pattaya on Saturday night.

November and December are among the most dangerous months for sea conditions in southern Thailand, and coincide with a rush of tourists seeking winter sun.