A British tourist is missing in Thailand after the tour boat she was travelling on caught fire off the island of Koh Tao.

Alexandra Clarke, 26, from Lambeth, south London, was onboard the Davy Jones Locker for a diving excursion.

She was reportedly in the bathroom when the blaze broke out, just before a distress signal was sent at 9.25am local time.

There were 21 other people onboard, including 15 tourists, two crew members and four diving instructors and assistants.

They were safely evacuated by private vessels and volunteers while a second team of volunteers worked to put out the blaze. Thai authorities then discovered Clarke was missing.

Capt Natthaphon Sinpoonphon, the deputy director of the Thai Maritime Enforcement Command Centre of Surat Thani, said the boat was between five and six nautical miles away from the island when a fire started in the engine room and “spread through the boat rapidly”.

He said: “Preliminary reports indicate that the engine room, captain’s cabin and rear restroom were damaged. The exact cause of the fire remains under investigation.”

It is understood rescue teams were initially unable to get close to the boat because of the flames, as there were fears of further explosions from fuel onboard the vessel.

Sinpoonphon said: “The search is continuing for one missing tourist, a female, Alexandra Clarke, from the United Kingdom.

“The conditions at sea are dangerous with the wind and currents. All boats in the area have been notified. Search and rescue teams were immediately mobilised.”

A spokesperson for the Foreign Office said: “We are supporting the family of a British woman who is missing in Thailand and are in contact with local authorities.”

A video shared on social media showed the boat engulfed in flames.

The Davy Jones Locker was reportedly transporting tourists from Koh Tao, a small island that forms part of the Chumphon Archipelago, to a nearby diving site at South-west Pinnacle.