Given its usual caution on travel advice, the guidance from the Foreign and Commonwealth Office on the maritime area in which the Chandlers disappeared is astonishingly vague.

Sailors are warned to keep at least 200 nautical miles (370km) away from the Somali coast. While reports of pirate activity in the Seychelles are noted, there is no advice for sailors. Ordinary tourists are comforted with the knowledge that attacks have only happened hundreds of miles from the main resort areas.

The confusion is in part due to the rapidly morphing threat from Somali piracy, which in the course of little more than a year has spread from the Gulf of Aden to the Indian Ocean waters off the southern Somali coast, and in the past few months all the way eastwards to the Seychelles and beyond.

Somali pirates began attacking vessels in the remote waters of the Seychelles’ Exclusive Economic Zone in April this year, seeking targets in new waters after being driven farther from their own coasts by the multinational force of warships patrolling the Gulf of Aden and the Indian Ocean.

The closest Seychelle island lies 670 nautical miles from the Somali coast. The area is lightly guarded, making it an ideal soft target for pirate skiffs operating from mother ships in the international waters of the Indian Ocean. When pirates first attacked their waters, the Seychelles coastguard had only one high-speed attack boat to its name, and that was in India for repairs.

The Seychelles appealed for help and India responded, sending a warship to patrol the archipelago, fearing that pirates might try to create bases in some of the scores of uninhabited islands. The Seychelles, in turn, deployed ground troops to the more remote islands.

Unlike the Gulf of Aden, the ocean area involved is too big to patrol effectively. In the Gulf of Aden, three different naval forces — one Nato, one European and one Combined Maritime Force including China, Russia and India — patrol a 500-mile long transit route that merchant vessels can follow in reasonable safety. Farther out on the high seas, around the Seychelles and north of Madagascar, foreign warships venture only when the pirates have already struck.

Last month the British-commanded EU Atalanta mission sent maritime patrol aircraft to be based in the Seychelles for everyday patrolling or to scramble when a pirate attack was under way. The United States deployed reconnaissance drones of the kind that, when armed, are used to target al-Qaeda and Taleban leaders in Pakistan, to provide an aerial eye.

The Seychelles coastguard has since seen off several attacks, giving chase to pirates in their skiffs. Yet in the past few weeks pirates have captured a fishing boat, a container ship and a cargo dry-bulk carrier in Seychelles waters. Last week they extended their reach even farther, seizing an Indian cargo ship 180 nautical miles east of the Seychelles.

Once pirates have boarded, the options are limited. Most commercial vessels will not permit any military action to be taken because of the risk to life and are willing to pay a ransom, or, more likely, the insurance premium that then covers it, to ensure that no one gets hurt.

However, the ransom option is unlikely to be available to the Chandlers, a couple in their fifties on a dream trip around the world. Just as the pirates’ reach has increased dramatically, so have their ransom demands, now running frequently into millions of dollars.

Last year President Sarkozy ordered a rescue mission by French commandos after a retired couple much like the Chandlers were captured aboard their boat by pirates demanding a €1 million ransom. It was a high-risk strategy that could have gone badly wrong. In April this year a young French yachtsman was shot dead when commandos stormed his boat in an attempt to release him and his family from Somali pirates.

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