This is the dramatic moment tourists scream with fright when a giant manta ray swims under their boat.

The holidaymakers had been on the speedboat visiting the Similan Islands in the Andaman Sea when the 18ft-long beast was spotted in the distance.

The mammoth marine creature began splashing at the surface around 100ft away before moving alarmingly quickly towards the vessel's starboard side.

Its huge body was seen as white outline lurking menacingly beneath the surface as it glided under the ferry and appeared on the port side.

A second clip then showed the manta ray circle back around the front of the speedboat, splashing around and swirling its tail, before disappearing into the depths of the ocean.

The heart-stopping footage was captured by a tour guide from Andaman Sea Sun Travel going to the popular Thai holiday islands on May 8.

They said: ''We saw the manta ray and everyone was getting a little bit excited, including me. I've never seen one before so it was quite an amazing moment.''