The government will investigate Future Forward leader Thanathorn Juangroongruangkit for possible involvement in an anti-Prayut Chan-o-cha demonstration in New York, after the party admitted he hired an American lobbyist firm.
Deputy Prime Minister Prawit Wongsuwon announced the investigation on Friday.
He was responding to the Future Forward Party's confirmation that Mr Thanathorn hired APCO Worldwide Co to facilitate his visit to Washington in July.
The party made the admission after media reports of an official document describing Mr Thanathorn's contract with APCO Worldwide, and a demonstration against Prime Minister Prayut in New York when he attended the United Nations General Assembly.
Although the Future Forward Party explained the hiring of the Washington-lobbyist, the government would launch an investigation anyway, Gen Prawit said.
The hiring could have an impact on Thailand if the contracted service concerned the country, because the employer was Thai, according to the deputy prime minister.
Gen Prawit said the anti-Prayut demonstration should not have happened during the prime minister's visit, and the demonstrators were not Thai.
The protest on Tuesday was recorded by Facebook user Chutima Liamthong. It shows demonstrators raising posters with messages calling for democracy in Thailand, and seeming to have little knowledge of Thailand.
Gen Prawit said he had no idea if the investigation would find any legal violation, and that it had nothing to do with the document on national sabotage that Gen Prayut was seen reading during the general debate in the parliament earlier this month.