Deputy Prime Minister Prawit Wongsuwon claimed on Tuesday that "all" of the 24 luxury watches that he has been documented wearing belong to his friends. He added that he would resign from the cabinet only if the national graft-busting agency decides there are sufficient grounds to pursue a case against him.

The nation's No.2 politician has for weeks been embroiled in a scandal over how he came to have so many expensive watches and other jewellery on the salary of a military man, and why he failed to include them in his declared assets.

The National Anti-Corruption Commission (NACC) has been looking into the case but critics claim the agency has been deliberately stalling in finding answers.

Tuesday's outburst by Gen Prawit marks the first time he has clarified where the watches came from after a group of "super sleuths" who contribute to the CSI LA Facebook account, which is hugely popular in Thailand, claim to have identified 24 different luxury watches on his person.

The group on Tuesday posted a photo of Gen Prawit wearing yet another luxury watch, identified as a Rolex Cosmograph Daytona IVORY DIAL worth an estimated 1 million baht (US$31,270).

Gen Prawit told reporters at Government House that he had borrowed the watches and only wore them occasionally, insisting he is not a collector.

They all belong to his friends and have since been returned, he claimed.

He refused to disclose how many watches he owns and threatened to resign if the NACC concludes he has committed any wrongdoing.

NACC secretary-general Worawit Sukboon said earlier the case was not complicated and the NACC should wrap up its investigation by the end of this month or shortly thereafter.

Gen Prawit has taken serious flak since pictures started circulating online of him wearing various luxury watches and other trinkets at public functions. None were included in the declared assets he submitted recently to the NACC.

Attention has turned to NACC president Watcharapol Prasarnrajkit, a former subordinate of Gen Prawit's.

Critics say this renders the investigation a sham due to the power of such connections in Thai politics.

Pol Gen Watcharapol dismissed worries and insisted a "clean" investigation would be conducted, adding that it was beyond his power to influence the decisions made by the other eight NACC members.

Gen Prawit has promised not to interfere with the investigation.

Nonetheless, Democrat Party leader Abhisit Vejjajiva has warned Prime Minister Prayut Cha-o-cha that the scandal could hurt his credibility and damage his chances of remaining in power after the election.

Meanwhile, unforgiving netizens have ridiculed Gen Prawit for what some have called a "lame" excuse that all the watches belong to his friends.

Calls have been growing that he reveal the names of the friends who allegedly lent him the watches.

Mana Nimitmongkol, secretary-general of the Anti-Corruption Organisation of Thailand, said he was stunned by Gen Prawit's remarks. He added that the NACC must forward the case to prosecutors if doubts remain over their findings.

"The NACC must summon witnesses and seek the truth about who lent them to him, where they bought them, how they afforded them and whether they paid their taxes.

"If there is any doubt, the case must be forwarded to the attorney-general and the anti-corruption court," he said.

Mr Mana said if the NACC is seen as prevaricating, members of the public or civic groups may file a complaint against it for dereliction of duty.

Srisuwan Janya, secretary-general of the Thai Constitution Protection Association, said Gen Prawit should resign from the cabinet.

"The public has already passed judgement on this case. If you insist on staying on, this will only hurt the image of Gen Prayut and the government," he said.

If the NACC takes Gen Prawit's remarks at face value then it serves no purpose and should be disbanded, he added.

But, he suggested, if it can prove his innocence it will earn the public's respect for acting transparently.