The last message that Paul and Rachel Chandler posted on their blog reads simply: “PLEASE RING SARAH”. Soon after, the British couple are believed to have triggered their yacht’s emergency-locator beacon in the Indian Ocean before being kidnapped by Somali pirates.

Mr and Mrs Chandler, 58 and 55, from Tunbridge Wells, Kent, were on a round-the-world journey and wrote their last, dramatic, entry on Friday night. No one has heard from them since. “Sarah” is thought to refer to Mrs Chandler’s sister.

Last night a pirate commander, Mohamed Shakir, said that his men were holding the couple captive. “We have captured the two old British, a man and woman, in the Indian Ocean. They were on a small boat we hijacked,” he told a Times correspondent in Mogadishu, Somalia’s capital, by phone.

Shakir, who runs three separate pirate crews from his base in Haradheere on the coast of Somalia, added that the couple were “healthy and in our hands”, but would not say where they would be taken when they reached the mainland. He said that a ransom demand would be issued.

The Chandlers left the Seychelles last Thursday in their 38ft yacht, Lynn Rival. They were heading for Tanzania via the Amirante Islands when their radio beacon began transmitting. Shakir said that a speedboat carrying a dozen pirates had spotted the yacht and that six had boarded the vessel.

Andrew Mwangura, the head of the East Africa Seafarers’ Assistance Programme, based in the Kenyan port of Mombasa, confirmed that Lynn Rival was missing.

HMS Cumberland, a Royal Navy Type-22 frigate, was alerted by the beacon and has joined the search for the vessel. It is one of five international warships looking for the couple, who were captured hundreds of miles from Somalia’s notoriously dangerous coastline.

The Foreign Office said that relatives of Mr and Mrs Chandler were being kept informed of developments.

“We are in touch with the family in the UK, and with the Seychelles coastguard,” said a spokesman. “They’ve conducted an initial search of the area and are continuing to investigate.”

Hopes rose last night that the couple’s vessel may have been spotted as dusk fell. Although it was too dark to see the name on the boat, an EU Naval Force (EUNF) helicopter was reported to have seen a yacht fitting Lynn Rival’s description, and it appeared to be towing two skiffs similar to those used by pirates.

“I don’t want to raise hopes but it’s the first sighting of a yacht that the EUNF has had so far,” Commander John Harbour told Sky News. “We’ve got two ships in the area and a helicopter, and we will keep looking.”

Before leaving the Seychelles, the Chandlers wrote on their blog: “We’ll be at sea for eight to 12 days, maybe 14 as we are now getting into the period of transition between the south and north monsoon, so the trade winds will be less reliable and we may get more light winds. We probably won’t have satellite phone coverage until we’re fairly close to the African coast, so may be out of touch for some time.”

In a previous blog post, the couple had written of “the Somali pirate problem” after two of their friends had delayed setting off from the Seychelles for fear of attacks.

Although deep in the Indian Ocean, Lynn Rival would still have been within range of the sophisticated pirate gangs that often use large “mother ships” loaded with fuel and supplies to launch sorties far away from the Somalian coastal waters where most attacks take place.

When they spot their prey, small groups of armed pirates set off from the mother ship aboard fast and light skiffs before boarding with extendable ladders and grappling hooks. They are commonly armed with AK47 rifles and rocket-propelled grenades.

With the monsoon season over, conditions have improved for the pirates. The lighter winds and smaller swell mean that it is both easier to see possible prey and board target vessels.

The presence of an international flotilla of warships has done little to deter the pirates. There have been at least 163 attacks so far in 2009, 47 of which were successful.

The most recent figures released by the International Maritime Bureau showed that attacks worldwide in the first nine months of 2009 exceeded the total number for 2008, itself a bumper year for piracy. “As with the figures of 2008, the lion’s share of hijackings have taken place off the coast of Somalia,” the bureau said.

Most of those kidnapped by Somali pirates are released unharmed, as long as a ransom is paid.

Somalia has had no functioning government since 1991 when the last one, a brutal military dictatorship, collapsed. The lack of law and order on land has allowed piracy to flourish at sea and, in recent years, the Somali coast has become the most dangerous in the world for piracy.

Elsewhere, pirate attacks are more bloody, if less frequent. In June, pirates operating off the coast of Thailand killed a 64-year-old Briton, Malcolm Robertson.

The search for the Chandlers is being co-ordinated by the UK Maritime Component Command in Bahrain. The hunt involves frigates from the EUNF, the multinational Combined Task Force-151 and Nato’s Task 508, all of which are involved in anti-piracy patrols across a vast stretch of sea from the Gulf of Aden to the Indian Ocean.

Additional reporting by our correspondent in Mogadishu

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