Prime Minister Prayut Chan-o-cha admitted Thursday that the government's "perceived undemocratic status" had a bearing on Thailand's latest fall in an annual global corruption index.

"That's how they perceived the government," Gen Prayut said. "But the result must be accepted."

When asked by reporters if the drop had anything to do with the luxury watch case involving Deputy Prime Minister Prawit Wongsuwon, Gen Prayut said, "It was only a trivial matter."

"Some major cases involving some fugitives were already closed. Why not look at those cases?" he said, without elaborating.

On Dec 27, the National Anti-Corruption Commission voted 5 to 3 to not pursue a case against Gen Prawit for failing to include 22 luxury watches and 12 rings on the list of assets and debts he submitted to the anti-graft agency upon assuming his cabinet post in 2014.

The commission found there was not enough evidence to rule the deputy premier deliberately concealed his wealth.

Thailand scored 36 points, a drop of one point from the previous year, on Transparency International's 2018 Corruption Perceptions Index, which was released on Tuesday.

The score ranges from 100 (very clean) to 0 (very corrupt).

Among the kingdom's Southeast Asian neighbours, Singapore was rated as the least corrupt with 85 points -- down one point from its 2017 score -- followed by Malaysia, whose score remains unchanged at 47 points.

Indonesia's score went down by one point to 38, while the Philippines' score went up by two points to 36 points. Vietnam's score went down by two points to 33, while both Myanmar and Cambodia lost one point each to score 29 and 20, respectively.

The worldwide average is 43 points.