It's an unusual move and no doubt his sympathy is genuine. Britain is second to Russia in the number of tourists that visit Thailand from Europe each year.
The message of condolences is addressed to Philip Hammond, who has been Britain's Secretary of State for Foreign and Commonwealth Affairs since July.
General Tanasak Patimapragorn, who was recently appointed Deputy Prime Minister as well as Foreign Minister, writes as follows:
I have learned with profound regret and sadness of the deaths of two British nationals, Ms Hannah Victoria Witheridge and Mr David William Miller, on Koh Tao on 15 September 2014. On behalf of the Royal Thai Government and the people of Thailand, I would like to extend to you and, through you, our sincere condolences and sympathy to the bereaved families for their irreparable losses.
I wish to reassure that the Royal Thai Government attaches highest priority to protecting safety of foreign nationals residing and visiting Thailand. On this particular case, General Prayut Chan-O-cha, Prime Minister of Thailand, has personally instructed the relevant Thai authorities to tighten security measures in the area. The instruction was also to conduct a transparent and thorough investigation into the causes and circumstances surrounding Ms Witheridge and Mr Miller's unfortunate deaths, with a view to arresting and prosecuting the culprits in the most expeditious manner. The Ministry of Foreign Affairs will work closely with the British Embassy in Bangkok and is ready to render assistance in any way.
Accept, Your Right Honorable, the assurances of my highest consideration.
General Tanasak Patimapragorn
Deputy Prime Minister
and Minister of Foreign Affairs of Thailand
Thailand has been under military control since General Prayuth took charge on May 22. European Governments have objected to military control and insisted on a return to democracy as soon as possible.
The letter of condolence has been posted today at mfa.go.th