Move Forward Party MP Rangsiman Rome on Monday accused Senator Upakit Pachariyangkun of making a false asset declaration after the senator filed a 100-million-baht defamation lawsuit against him last week following the general debate.
During the debate in which the government was grilled for its failure in tackling illegal drugs, Mr Rangsiman raised the question of money laundering allegations against a senator with the Thai initial "Or".
After the debate concluded, Mr Upakit had his lawyer file a defamation lawsuit against Mr Rangsiman, seeking 100 million baht in damages.
Undeterred, the opposition MP on Monday held a press conference in which he suggested that Mr Upakit might have falsely declared his asset upon assuming his senatorial post.
The MP said he suspected that the senator did not sell a hotel business worth US$8.15 million (280.1 million baht) as claimed. This raised the question of the source of the $8.15 million that appeared in the senator's bank account.
During the debate, he said if the hotel sale was a cover-up for an activity forbidden to senators, he would lodge a complaint with the National Anti-Corruption Commission and suggested the land may belong to "Or".
Mr Rangsiman also demanded an explanation from Prime Minister Prayut Chan-o-cha as to why investigators in charge of the drug trafficking and money laundering case against Tun Min Latt, a well-connected Myanmar national, were transferred.
Tun Min Latt was arrested in September last year along with three Thai nationals. Mr Upakit denied having any close relationship with the Myanmar tycoon after the latter's arrest.
PM's Office Minister and UTN member Thanakorn Wangboonkongchana insisted on Monday that the party and Gen Prayut, who is a party member, have no involvement with any grey businesses. He said the party does not want to file lawsuits because the general debate is part of the checks-and-balances mechanism. However, it will not tolerate groundless accusations, he added.
Meanwhile, Mr Upakit on Monday had his lawyer file a defamation lawsuit against Danai Akemahasawat and Amornrat Mahitthirook, hosts of the Inside Thailand television news programme, seeking 50 million baht in damages, for insinuating he was involved in illegal activities.
Ruangsak Suksaengsri, the senator's lawyer, said three lawsuits had been filed so far by Mr Upakit to protect his name. The other is against Atchariya Ruangrattanapong, chairman of the Crime Victims Assistance Club.