Prime Minister Prayut Chan-o-cha lashed out at a key member of the government's anti-corruption drive Tuesday who had urged him to take action over the luxury watch scandal involving Deputy Prime Minister Prawit Wongsuwon.

The premier's outburst came after Totrakul Yomnak, chairman of a sub-committee against corruption, sent him a letter expressing concern about the watch scandal.

Gen Prayut chairs the National Anti-Corruption Committee.

According to Mr Totrakul, the issue was undermining public confidence in Gen Prayut and the committee formed by the National Council for Peace and Order (NCPO) to tackle corruption.

He called on the prime minister to take action and show his determination to address graft, which he has declared a top priority.

However, Gen Prayut replied that the luxury watch scandal was already being investigated.

"Don't make it over-complicated or stir up problems," he said. "Let the scrutiny mechanism do its work. And if wrongdoing is established by the probe, the case will go to court."

Pressure keeps mounting on Gen Prawit. Last Friday, former National Reform Council member Thicha Nanakorn submitted an online petition with more than 80,000 signatures urging Gen Prayut to drop his deputy from the cabinet.

The names were gathered on change.org from Jan 31-Feb 15 to pressure Gen Prawit into resigning. He said last month he would step down if the people demanded it, but then indicated he had changed his mind.

In a related development, Gen Prayut shrugged off the latest findings on corruption by the University of the Thai Chamber of Commerce, which said corruption was worsening under the current regime.

He said a survey like that could hurt Thailand's reputation and scare off investors.

He later clarified that this did not mean information tied to corruption scandals should be withheld.

The survey, released last week, showed that Thailand's place on the Corruption Situation Index (CSI) worsened between June and December.

It questioned 2,400 Thai residents, businesspeople and government officials nationwide in December. The CSI survey is conducted every six months.

According to the feedback, corruption may spread this year because state officials were seen as having too much discretionary power under the law.

A lack of political transparency and ineffective law enforcement were also cited as opening the window for more graft.