Prime Minister Prayut Chan-o-cha has fully defended his deputy Gen Prawit Wongsuwon who is under heavy public pressure concerning his collection of luxury wristwatches.

He insisted the case was "a personal matter" and he was not required to step in at this time.

"You may forget that this is a personal matter... a personal flaw," he said Tuesday.

"There is already a law to deal with this. It should be left to the anti-graft agency to handle," Gen Prayut added.

He was responding to criticism from the public that the premier has taken no action in relation to the snowballing scandal.

The National Anti-Corruption Commission (NACC) will announce any progress of its probe and the public must be patient, he said.

"It would be a different matter if it involved any misspending of the state budget. Don't mix the two issues up. Just let the NACC investigate according to legal procedures," Gen Prayut said.

He insisted he was not defending Gen Prawit or trying to conceal anything.

Gen Prawit previously said he was ready to resign if the national graft-busting agency decides there are sufficient grounds to pursue a case against him.

Gen Prayut said he was not employing "double standards" by failing to exercise his special power granted under Section 44 to suspend Gen Prawit from duty despite doing so for other officials whose reputations have been tarnished.

"I used Section 44 to punish others because the agencies that probed them recommended they be suspended. That doesn't mean I can just use it with anyone. In this case, we need to see if that's the NACC's judgement."

The NACC launched its probe following public criticism after suspicions emerged that Gen Prawit had failed to report over 20 expensive watches among his declared assets and liabilities as required by the anti-graft agency.

The popular CSI LA Facebook page has spotted at least 25 luxury watches worth about 39.5 million baht on the deputy premier's wrists.

Gen Prayut dismissed speculation that Gen Prawit will resign. A source at the NACC said its secretary-general, Worawit Sukboon, will hold a briefing today to update the media on the probe.

Mana Nimitmongkol, secretary-general of the Anti-Corruption Organisation of Thailand, said the matter cannot be regarded as personal given that both the prime minister and deputy prime minister are public office-holders who exercise state power.

The public wants and has a right to know if they are honest, Mr Mana said.

They are not interested in his personal taste but whether he earns enough money to afford all these luxury watches, he said.

Democrat Party leader Abhisit Vejjajiva insisted that Gen Prawit should come forward and explain himself.

Mr Abhisit said earlier the issue has undermined the public's confidence in Gen Prawit and the regime, making the government's work harder and its popularity suffer.

Gen Prawit claims all of the expensive watches he was photographed wearing belonged to his friends and have since been returned.