Deputy Prime Minister Prawit Wongsuwon must resign before his reputation is ruined by the ongoing saga over his collection of luxury wristwatches, former deputy prime minister MR Pridiyathorn Devakula said Monday.

"With the situation being this tense, if I were him, I'd resign," MR Pridiyathorn said in an interview with the investigative current affairs programme Jo Luek Thua Thai Inside Thailand on Spring News.

Gen Prawit is being investigated by the National Anti-Corruption Commission (NACC) after the popular Thai-language CSI LA Facebook page published photos showing him wearing 25 timepieces worth a combined 39.5 million baht (US$1.24 million).

This prompted critics to question how he was able to afford them on a military salary and why he failed to list them among his declared assets.

Gen Prawit has said they were all borrowed from friends and have since been returned, but this excuse has been widely ridiculed.

MR Pridiyathorn said if a majority of the public finds the scandal unacceptable, Gen Prawit would face a difficult road ahead that could leave his reputation in tatters.

Gen Prawit has not yet been charged with any form of misconduct or corruption. If the NACC finds him guilty of not declaring his assets, that would not automatically be treated as corruption but rather as a violation of the regulations pertaining to assets held by political office holders.

"But if he stays on and the people have lost trust in him, his reputation will be irrevocably damaged," MR Pridiyathorn said.

Some reports, quoting a former military officer who spoke on condition of anonymity, claim businessman Pattawat Suksriwong, who died last year, would often lend the deputy prime minister watches.

Gen Prawit and Pattawat were close friends as they attended Saint Gabriel's College together, according to Gen Noppadol Inthapanya, a member of the National Reform Steering Assembly (NRSA).

Mr Pattawat was a very wealthy man who frequently let Gen Prawit borrow his watches, Gen Noppadol claimed.

The two men were part of an elite circle of friends who liked to swap watches temporarily, he said.

"Even I borrowed one from Mr Pattawat, gave it back to him after two months and got a new one from him on loan," he said. "Prime Minister Prayut Chan-o-cha also knew Mr Pattawat well."

A source close to Gen Prawit said the scandal has put him under immense pressure and caused him to seek medical attention.