Somali pirates have seized a high-class dive boat operating in the Seychelles archipelago, in the Indian Ocean, officials said. A group of British tourists returning from a diving expedition narrowly avoided being kidnapped.

The British tourists had gone ashore at Assumption Island only hours before the Indian Ocean Explorer and its seven-man crew, all from the Seychelles, were seized. The 115ft cruiser is a former oceanographic research ship.

Kirk Green, the director of Aquatours, the London-based tour operator that books out the yacht, said that the Royal Navy had e-mailed to inform him that the vessel had been hijacked. He was told that the boat would be taken to Harardhere, a pirate stronghold north of Mogadishu and that he should expect the vessel to be held there for about three months.

Last week, the islands’ police chief said three Seychellois sailors had been held hostage by Somali pirates since their catamaran was hijacked in late February.

An international task force, including ships from the Royal Navy, has been patrolling the waters close to Somalia since January, after more than 100 vessels were hijacked last year. This has reduced the number of successful boardings in the Gulf of Aden, but the pirates are now venturing much deeper into the Indian Ocean. The Aldabra islands lie more than 700 miles south-east of the Somali coast.