A BRITISH diving instructor is missing, presumed drowned, after failing to return to the surface from a deep dive with a party of tourists at a site off Koh Tao, in the Gulf of Thailand.

Rescuers have so far failed to find diving instructor Peter Joseph McCarthy, 47, who vanished on a dive to 80 metres onto a submerged volcano mouth with a party of eight other British and Italian divers about noon on Monday.

David Powley, 42, the other instructor on the dive, told Thai media that Mr McCarthy did not come up after a one-hour dive undertaken by all nine divers.

Each of the divers had two hours of oxygen. The other divers used up their remaining oxygen in searching for Mr McCarthy, reports said.

On Wednesday, according to police in the nearby province of Surat Thani, three divers searched for Mr McCarthy from 8am and four more divers failed to find him in a second dive about 10.30am.

Another search will be undertaken today.

''It's very deep water,'' Lieutenant Paisan Srisawat told Phuketwan by telephone from Tung Yai police station, in the nearby province of Nakkornsrithammarat. ''The region is beautiful but divers have to dive carefully to make sure there are no problems.

''These two instructors had plenty of experience in the region.''

The original diving group consisted of the two instructors, four men and three women. Mr McCarthy had a diving licence issued in the Gulf of Thailand province of Chumpon.

Divers usually reach Koh Tao from two better-known islands, Koh Phangan or Koh Samui. Visitors mostly fly to Koh Samui and then travel by boat to Koh Tao or bus/train to Chumpon, then take the fast ferry to Koh Tao.