The final ruling, which ends a trial that lasted almost a decade, will be sent to Anantasak, 50, at Khao Bin Prison in Ratchaburi, where he is detained.
The lawsuit said the three defendants and Sangad Pumpeng - who died later - staged a deadly ambush on May 27, 2006, killing Kobkul while she was in her car at Khao Ngu intersection in Tambon Chedi Hak in Ratchaburi's Muang district. The other two persons in the car were injured.
Anantasak and two others - snooker club employee Anont and vegetable vendor Winyu Rattanawannee - were later arrested. An initial court ruling on March 31, 2011 gave the three death sentences but commuted punishment for Anont and Winyu to life in prison because they confessed. The Appeal Court upheld the punishments for the trio, so Anantasak, who claimed he was innocent, appealed again.
Anont also appealed for lighter punishment but withdrew his request later. The Supreme Court upheld his life sentence.
Kobkul's husband, former deputy public health minister Manit Nopamornbodi, said yesterday he was pleased with the verdict and thanked police for working hard and gathering forensic evidence so that all defendants could face the maximum punishment.