British Airways has agreed to codeshare on a series of routes with Bangkok Airways.
The agreement covers four routes between the Thai capital Bangkok and Singapore, Koh Samui, Phuket and Chiang Mai.
British Airways customers will be able to book flights, via Bangkok, to these destinations in one transaction on ba.com.
BA flies daily to Bangkok and twice daily to Singapore. The Bangkok service will be moving from Heathrow Terminal 3 to Terminal 5 from October.
Steve Ronald, British Airways' head of alliances, said: "Thailand will be more accessible to European tourists now that we have teamed up with Bangkok Airways.
"The move of our Bangkok service to Heathrow's Terminal 5 will also serve to streamline connections between Europe and the Far East."
Bangkok Airways was originally founded as an air taxi service in the 1960s and it has now grown to serve a network of 20 destinations with a fleet of 22 aircraft.
Peter Wiesner, the carrier's senior vice president network management, said: "This new partnership will offer smooth transit for passengers travelling from long-haul routes with British Airways to Bangkok and Singapore for their onward journey to Thailand's key destinations."
Last month, Bangkok Airways and Qatar Airways agreed to codeshare seven domestic routes within Thailand and two international routes.
ba.com
bangkokair.com