Thanat, Philippine foreign secretary Marciso Ramos, Singapore foreign minister Sinnathamby Rajaratnam and Malaysian deputy prime minister Tengu Abdul Razak signed the document establishing ASEAN on August 08.
Sompong said a discussion was held on August 06 and two ministers from Sri Lanka (then Ceylon) were in an adjacent room.
"I remember one was an economics minister. He waited there anxiously for a signal to join the discussion; but it never came," Sompong told The Nation
"It was Rajaratnam of Singapore who opposed the inclusion of Sri Lanka."
"He argued the country's domestic situation was unstable and there would be trouble. Not good for a new organisation," Sompong recalled.