BANGKOK, Oct 16, 2009 (AFP) - Thailand's Supreme Court Friday 16 October upheld a death sentence for a former senior policeman who abducted and murdered a gem merchant's wife and son as he investigated the theft of Saudi royal jewellery.

Ex-police commissioner Chalor Kerdthes was convicted of the abduction and murder in 1994 of the family members of Santi Srithanakhan, who had bought some of the jewellery stolen in a notorious heist from a Saudi prince's palace.

The prosecution argued that Chalor abducted the pair to pressure the merchant into revealing the whereabouts of the lavish gems, worth 20 million dollars.

Chalor, 71, was initially sentenced to life imprisonment, but an appeal court in 2006 upped his penalty to the death sentence, leading him to further appeal to the highest court in Thailand.

"The Supreme Court upholds the appeal court's decision on the death sentence as his appeal was groundless," a court official said Friday 16 October.

Chalor could still submit a petition to the Thai king seeking royal amnesty within 60 days, a corrections department official said.

The former policeman was also handed a 20-year prison sentence in 2006, while on death row, for stealing some of the spectacular jewels he was supposed to be recovering.

A Thai janitor was convicted and imprisoned for stealing the gems in 1989 from the Saudi palace where he worked, before smuggling them back to Thailand.

Police recovered most of the jewellery but much of what was returned to the Saudi owner proved to be fake and many pieces are still missing.

The gem heist was also linked to the disappearance of Saudi businessman Mohammed al-Ruwaili in 1990 and the unsolved murders of four Saudis, including three diplomats, in 1989 and 1990.

Thai police have never tracked down the killers of the Saudi diplomats and the unsolved murders have long-plagued relations between Thailand and Saudi Arabia.